Setup for the Analysis of Nucleate Boiling. On the Surface of a Heated Rod

Transcription

1 Setup for the Analysis of Nucleate Boiling On the Surface of a Heated Rod Nadim Vira A thesis submitted in conformity with the requirements for the degree of Master of Engineering Graduate Department of Mechanical and Industrial Engineering University of Toronto Copyright by Nadim Vira 2010 Page 1 of 51

2 Acknowledgements I would like to start by thanking our Professor Nasser Ashgriz, for his continuous support in the M.Eng Program. I would also like to thank our colleagues Reza Karami, Farhan Sultan and Amireza Amighi for their contributions in the project. Their combined knowledge in thermodynamics, fluid mechanics and general machining was invaluable throughout the life of the project. A special thanks to those who were not directly involved with the project but whose advice and suggestions helped the team make important decisions. I would like to thank Professor Schmid for his insight in Computer Aided Design, Ryan Mendell from UofT s machine shop for his help with machining and Michael Romanovski from GE Waters. Last but not least, I would like to acknowledge the contributions of Lionel Lobo, my project partner and fellow Masters student at the University of Toronto. His continuous support and knowledge of fluid mechanics made this project a success. Page 2 of 51

3 Abstract Engineers designing nuclear reactors use conservative parameters in order to accommodate the large unpredictability in the heat transfer process in nuclear fuel channels. This unpredictability can be attributed to insufficient experimental data on nucleation on the surface of nuclear fuel rods. A number of studies have been performed to develop formulas and techniques for measuring parameters such as bubble diameter, velocity and frequency in order to properly understand how these properties may affect heat transfer. In an effort to further study these characteristics, an experimental set-up was created to mimic a nuclear fuel channel. Conditions in a nuclear plant were scaled down to create an environment that made the study of nucleation feasible. Inlet water pressure and flow rate were varied in this setup and the effects of these variations on nucleation were observed photographically. A significant setback faced during this project was related to the design and installation of the glass windows. Properties of quartz and borosilicate were studied and recommendations for future projects were presented. Due to time and design constraints, conclusive results for bubble diameter and velocity were not possible. A sample reading was analyzed to measure the bubble frequency. At the end of the project, a set-up was presented and techniques for studying nucleation were documented. Page 3 of 51

4 Table of Contents Acknowledgements... 2 Abstract Introduction Literature Review Nuclear Fuel Rod System Parameters Nucleation Glass Types Borosilicate Glass Fused Quartz (Silica) Glass Structure and Effects of Cracks Design Experimental Setup Results and Discussion General Recommendations Bubble Departure Diameter Bubble Velocity Bubble Frequency Future Recommendations Conclusion Appendix References Page 4 of 51

5 List of Symbols or Abbreviations A c C PL d d W E f F G Unsupported Area Crack depth Specific heat capacity of the liquid Bubble diameter Bubble departure diameter on the wall Young s modulus Bubble departure frequency Safety factor Gibbs free energy. m L Mass flow rate of water (kg/s) m W M n Nu P Pr S f T T L Evaporation rate Modulus of rupture Polytropic exponent Nusselt number Pressure Prandtl number Fracture stress Glass thickness Local liquid temperature Page 5 of 51

6 T sat T w Saturation temperature Wall temperature T W,ONB Wall temperature at which onset of nucleate boiling occurs u W Friction velocity Z ONB Location of ONB point ρ G, ρ V ρ L Vapour density Liquid density µ l Liquid viscosity Page 6 of 51

7 1.0 Introduction In order to properly mimic a fuel rod cooling system it is important to understand what the system is used for and how characteristics of coolant flow will affect the final product i.e. production of electricity. Fuel rods in nuclear power plants must be cooled at all times to ensure safe operation. To do this, the surface of these rods are constantly surrounded by a layer of pressurized flowing water, which while acting as a coolant is also responsible for acting as an agent of energy transfer. Fission reaction in the reactor core releases a massive amount of heat that is used to heat this pressurized water and convert it to steam. The steam is then used to produce electricity. In a laboratory setting, it is almost impossible to mimic such conditions without spending a large sum of money to create a nuclear friendly laboratory, safe equipment, trained staff and access to the nuclear material. For this project, the requirements were scaled down and the physical and thermal characteristics can only be used as a guideline for the development of a safe experimental set-up. The objective of this research project is broken down into two parts: (1) is to design a scaled model of a nuclear fuel channel conducive of nucleation and (2) is to study the characteristics of the bubbles formed due to nucleation on the surface of a heated fuel rod. The results obtained by studying characteristics such as bubble frequency, bubble diameter and initial bubble velocity can be used in future studies on more complex models and at higher temperatures and pressures. Page 7 of 51

8 2.0 Literature Review 2.1 Nuclear Fuel Rod System Parameters Commercial nuclear reactors are large and operate at extremely high temperatures and pressures, ranging from around C and Mpa respectively. Power output of the entire reactor is determined by the number of fuel channels in the core of the reactor. Each CANDU reactor can contain between 380 to 480 horizontal channels and each of these channels consists of a pressure tube which contains the fuel rods, heavy water coolant and outer Calandria tube 1. Fuel channels are arranged in rows and columns on a square lattice pattern at a standard pitch of 11.25, forming an approximately circular array 2. Each fuel rod is approximately 19.5 long and 0.39 (1cm) diameter. Figure 1: Fuel Rods, Fuel Bundles, Pressure Tube and Calandria 1 A kilogram of U-235 converted via nuclear processes contains approximately 3,000,000 times the energy of a kilogram of coal burned conventionally. The coolant being circulated Page 8 of 51

9 has to be able to effectively carry away this massive amount of heat. For this reason the coolant comes in at a high pressure and velocity. Figure 2: Nuclear Power Plant Schematic 2.2 Nucleation Nucleation can be defined as the onset of a phase transition in a small region. Formation of these sites can be attributed to imperfections in the heated tube and presence of suspended particles and minute bubbles in the cooling water. This is called heterogeneous nucleation. Nucleation without preferential nucleation sites is called homogeneous nucleation. Nucleation sites are normally present on the outer surface of the heated tube which is in contact with water. Page 9 of 51

10 2.3 Glass Types In order to determine the material to be used in for the glass windows a study of different types of glasses was necessary. Initial research led to 2 viable candidates: Borosilicate Glass or Fused Quartz, and a comparison between these candidates were required Borosilicate Glass 3 This type of glass has been developed specifically for use in laboratories and applications with harsh thermal, mechanical and chemical conditions. Common names include Pyrex or Borofloat. The main component of borosilicate is boron with SiO 2. Advantages: Easier to hot work than quarts making fabrication less expensive. Lower material cost than fused quartz Lower coefficient of expansion than all glasses except fused quartz. It can therefore be used in high temperature environments without the risk of breaking due to thermal shock Good optical quality Easier to mould to required shapes than quartz Thermally stable to 450 C for continuous use and 600 C for short periods Disadvantages: Will get deformed if exposed to temperatures greater than 450 C for long periods Page 10 of 51

11 Coefficient of expansion is 60 times less than fused quartz Cannot be fully tempered like regular soda lime glass Fused Quartz (Silica) 3 This is an amorphous form of quartz which is made by purifying and melting natural crystalline quartz. Advantages: More shock resistant than other types of glass and very low coefficient of expansion Best transmission characteristic among glasses Best thermal characteristics among glasses. Thermally stable to 900 C to 1100 C for continuous use and 1400 C for short periods Very low thermal conductivity Can be ground and polished and molded into any shape Harder than most other types of glasses Disadvantages: Much more costly than Borosilicate High fabrication cost due to high melting point 2.4 Glass Structure and Effects of Cracks In crystalline solids, such as metals, linear dislocations in the otherwise regular atomic packing enable planes of atoms to slide over one another under very low shearing stresses. Page 11 of 51

12 Sliding of planes of atoms also occurs under applied tensile stresses as planes at an angle to the tensile axis experience a shearing stress. Therefore, stresses exceeding the yield point initially lead to plastic deformation rather than fracture. By contrast, in a brittle material such as glass, there are no regular planes of atoms on which sliding can occur, thereby eliminating the possibility of plastic deformation. As a result, at temperatures below melting point, mechanical fracture of glass is caused by brittle fracture which mainly occurs by the initiation and propagation of cracks 4. This means that glass is severely weakened by sharp notches or cracks. It has been proven that the stress on the material close to the tip of a crack is much higher than the stress on the whole specimen. As a result bonds are easily broken at crack tips and can easily propagate. Studies have shown that cracks around 10-3 mm long can decrease the strength of glass by a factor of Separate tests have also shown that failure stress is dependent on the square root of crack length 5. Griffith derived the equation: 2 2Eγ S f = π c Where, S f is fracture stress, E is Young s modulus, c is crack depth, γ is surface energy Page 12 of 51

13 3.0 Design This section of the report contains all the details about the design of the experimental set up, including set-up failures, solutions to these short comings and precautionary measures for future set ups of this nature. The objective of the experimental set up is to develop a scaled down version of a nuclear fuel rod system and to successfully create an environment that supports nucleation. The purpose for this study is to relate nucleation to changes in pressure and flow rate keeping inlet water temperature constant. The set-up is broken down into three main subassemblies: The tank assembly, the chamber assembly and the camera set up. This set up will allow users to vary various parameters to analyze bubble diameter, velocity and frequency and find relationships between nucleation characteristics and thermodynamic properties. 3.1 The Tank Assembly The tank assembly, although simple, required constant modifications and improvements to help properly introduce the right conditions of water into the systems. Conditions such as transparency, saturation temperature, and outlet flow were a few requirements to the tank set up. The main components of the tank assembly include an insulated heated tank, a pump and a temperature control device. The tank assembly has one major objective, that is, Page 13 of 51

14 to provide a reservoir of heated water for the nucleation chamber. The components of the tank assembly allow us to maintain a uniform water temperature throughout the tank. Figure 3: Schematics of the Tank Assembly, depicts the various instruments and components that maintain the functionality of the tank assembly. The tank is first filled with filtered tap water at around 21 C. Once filled the built in heater is used to raise the temperature of the water to the required temperature. During this time the pressure release outlet is kept open top maintain atmospheric pressure in the tank. The rise in temperature is monitored with the help of a K-Type thermocouple attached to the outlet of the pump. The pump is used to circulate the water within the tank maintaining a uniform temperature throughout. The thermocouple connected to the data acquisition system keeps track of the temperature in the tank and regulates power supplied to the heater. Once programmed, it maintains a desired temperature by turning the heater ON/OFF. After the desired temperature has reached, compressed air is introduced into the tank to push the water out of the tank and into the chamber. The pressure of the tank is monitored using the pressure gauge and a maximum of 100 psi is used. For safety, a pressure relief valve was inserted to release pressure exceeding 100 psi. At this point, both the heater and the pump are turned off. Previous trials show that the temperature does not drop quickly over time due to the insulation surrounding the tank. During the experiments, the temperature in the tank is regularly measured. When ready, the outlet valve is opened allowing the water to flow into the chamber assembly through a stainless steel metal hose. Trial and error showed that the rusting and sediment settlement in the tank over the life of the tank could pose a possible problem during the imaging stage of the experiment. In order to avoid such Page 14 of 51

15 issues, a nano filter was initially introduced to pick up all the particles in the water. Unfortunately, the amount and size of the particles were too large for the filter causing it to clog. Therefore a bronze y-shaped strainer was chosen as a better alternative. The mesh in the strainer can catch larger particles and is easy to clean. In addition to the above precaution, CLR was circulated through the tank in order to remove the pre-existing rust and to clean the tank as much as possible. As recommended, CLR was mixed with warm water in the tank and continuously circulate for approximately 4 hrs. The circulation is necessary to introduce turbulent flow into the system that will help erode rust from the inside surfaces. Due to the varied types of metals in the system a more specialized chemical was deemed unsuitable for this set up. In the future, this set-up would be more effective if the materials used were kept constant to one type of metal stainless steel as recommended by a professional from GE Water Michael Romanovski. Once the experiments have been completed the pressure release valve is used to lower the pressure in the tank to atmospheric. It is recommended that the tank be completely drained after every use to avoid rusting. A set up time of about 15 minutes is needed to fill the 30 gal tank and a little over two hours to heat the water to around 90 C. Below are the schematics of the setup followed by a picture of the current set-up. Page 15 of 51

16 Figure 3: Schematics of the Tank Assembly 3.2 The Chamber Assembly This is the part of the assembly where the nucleation happens. As shown in the figure below, the assembly consists of a combination of instruments, a chamber with a window and a cartridge heater. The chamber assembly is designed so as to be able to easily monitor and vary the pressure, temperature and flow properties of the system. Its main aim is to create an environment conducive to nucleation. The chamber is equipped with windows that allow the recording of slow motion videos and photographs of bubbling using a high speed CCD Camera in order facilitate calculation of the size, velocity, location and frequency of nucleation. Page 16 of 51

17 The assembly was designed to work as follows: The heated water from the tank would be allowed into the chamber by opening the ball valve located in the beginning of the assembly. The more the valve was opened the higher the flow rate. A flow meter was also attached in the inlet to allow for controlled flow. To study nucleation properly, the flow rate, temperature and pressure would all need to be recorded and its relationship to the properties of nucleation would be analyzed. Before entering the chamber, a thermocouple measures the temperature of inlet temperature. This temperature is one of the control variables used to study nucleation. The pressure gauge adjacent to the thermocouple measures the gauge pressure in the chamber. As the outlet valve is slowly closed the pressure increases in the chamber. Pressure directly affects the saturation point of water; therefore, varying the pressure should vary the number of nucleation sites, the nucleation frequency and whether or not nucleation happens. The Nucleation Chamber is designed to create an environment conducive for nucleation imaging. The key objective of this chamber is to raise the temperature of the water from just under saturation to saturation at given pressure, flow and temperature conditions. In addition to that, imaging of the bubbles is necessary in the study of the properties of nucleation. To accomplish this, the device must have windows on two sides to allow light to pass through. Further instruction on the proper use of light and camera will be discussed in the following section The Camera Assembly. Page 17 of 51

18 Figure 4: The Schematics of the Chamber Assembly The requirements of the project were as follows: psi pressure, C temperature, low turbulence, m/s inlet water velocity and a heat resistant glass window to allow for imaging. The dimensions of a nuclear fuel channel were used as the basic design platform. That is, a horizontal cylindrical fuel rod submerged in a chamber with flowing water. Since the energy output of a nuclear fuel rod could not be mimicked the most powerful cartridge heater was purchased and scaled down pressures, temperatures and flow properties were calculated as shown in the calculations section. A custom made Tempco cartridge heater was purchased from Process Heaters (A distributor for Tempco Electric Heater Corporation). The specifications were as follows: Page 18 of 51

19 Specification: Dimensions: 0.496" OD ±.002" x 23.22" ± 2% Voltage: 240 VAC. 1ph Power: 2500 watts (+5%, - 10%) Sheath: Termination: Thermocouple: 321 stainless steel and will be polished CMR termination, stainless single threaded 3/8-18 NPT fitting silver soldered to sheath Built-in Type K ungrounded thermocouple - 30 mm unheated at disk end, 60 mm unheated at lead end Others: centre 500 mm is heated length (approx wsi). - Junction centred in heated length and in centre of diameter. - 48" fibreglass insulated power and thermocouple leads. As mentioned above the cartridge heater is sectioned into three parts: the two cold sections in the front and end to ensure that the restraints did not get heated while the middle hot section was left suspended in the water with no obstructions. Figure 5: Type CM - Single Threaded Screw Plug Cartridge Heater The middle section of the chamber is comprised of a hollow rectangular aluminum block with slots cut out for the glass window and flanges. Two separate components were designed for the front and back end of the chamber. The front section consists of an NPT 3/8 thread for easy attachment of the inlet piping. The back section also includes the thread Page 19 of 51

20 for piping but an additional ½ thread was added for the cartridge heater assembly. All metallic components were manufactured using Aluminum The Solidworks models and pictures attached provide a better idea of the chamber assembly. During the life of the project, there was a significant amount of redesign required to make this chamber work. Some of the concerns stemmed from the machinability of the components and costs while others stemmed from component failures and safety. One persistent problem came from the components used to restrain the glass. Throughout the length of the experiment the team frequently ran into the problems with cracking or shattering glass. Two types of glass were used at different times in the project, Quartz and Borosilicate. The following few sections will discuss the three major design changes, their implications and the lessons learned Initial Flange Design The window was designed using a ¼ quartz glass held in place by ¼ flanges on either sides of the chamber as illustrated below. This design faced various problems and given below are the modifications made to troubleshoot these problems. Figure 6: Design 1 Nucleation Chamber Page 20 of 51

21 Figure 7: Exploded View - Design 1 Troubleshooting: As part of the troubleshooting process, two components were identified to have been the cause of failure. The first was the quartz glass and the second was the deformed flange (as shown in the picture below). Before assembling the components, the flange was designed to be the clamping mechanism for the glass. This would theoretically provide uniform pressure over the glass and would be easy to assemble. It was understood that there would be slight deformation/buckling in the flange but that this was not identified as being able to add enough stress to the glass. In addition to the buckling, a torque of 40ft-lb was added to each bolt securing the glass in order to prevent leakage. In retrospect this amount of torque was found to be well above the value needed to prevent leakage. The coupled effect of the excessive torque and the uneven loading caused by buckling of the flange created high stress areas on the glass and both pieces of glass showed similar crack patterns. Page 21 of 51

22 Figure 8: Flange Buckling Front View Figure 9: Flange Buckling - Side View When the design was analyzed in CosmosWorks it was found that the deformation in the shape of the flange created excessive forces on the edges of the glass. Due to the deformation in the flange, the bottom face of the flange was not in uniform contact with the glass surface, leading to non-uniform pressure being applied on the face of the glass. In fact, if closely analysed, most of the glass- metal contact occurred at the edges of the glass rather than on the face subjecting the edges of the glass to carry the entire pressure of the clamp. This increased stress coupled with the non-uniformity created vulnerable spots on the glass. This effect of this stress is magnified by the lack of bolts at the corners which allows more deformation. Combinations of all these factors led to deformation of the shape Page 22 of 51

23 as shown in the diagram below. This resulted in the glass breaking at a specific spot shown in figure 13. Figure 10: CosmosWorks FEA Simulation of Deformation of Flange Figure 11: Glass Cracks in a Straight Line Page 23 of 51

24 To avoid a similar outcome it was important to redesign the flange so that it would minimize the amount of deformation. Additionally, the excessive torque used to tighten the flange needed to be re-evaluated and reduced. At this point in the project, time and budget constraint led us to analyze one more component in the assembly the glass. Similar to the glass used in the initial design, Borosilicate exhibits comparable thermal properties. It can withstand high temperatures as per our requirements; it costs almost 3 times less and could be delivered within 2 days (compared to the 6 weeks for quartz). The tradeoffs seemed acceptable and the change from Quartz to Borosilicate was made in the revised design Revised Flange Design: After learning from the mistakes made in the initial design, one major modification seemed critical to the success of this project. The previous design allowed too much room for buckling adding uneven loading on the glass. A seat style flange redesign would allow the glass to sit within the seat and provide uniform loading on the glass. This redesign ensured minimal buckling while providing a snug seat for the glass. This coupled with the reduced torque would reduce the pressure on the glass significantly. The design below was the proposed design: Page 24 of 51

25 Figure 12: Design 2 - Nucleation Chamber Figure 13: Exploded View - Design 2 All components of the initial design were kept the same other than two parts, the flanges and the glass. As mentioned in the section above, the trade-off between borosilicate and quartz seemed tolerable. On the other hand, the redesign of the flange and the reduced torque promised lower pressures on the new glass. Although the lower yield strength of borosilicate makes it a riskier material to use, the reduced pressures were well below the Page 25 of 51

26 yield point of the glass. With recommendations from the glass shop in the chemistry department a local manufacturer was contacted for to make two pieces of borosilicate with identical dimensions as the quartz in the initial design. Once the components were assembled, the changes looked promising. The new design did withstand the new torque of 20ft-lbs and there were no signs of cracks. This provided a good opportunity to run a quick test with water. Having considered proper safety measures, warm water at a pressure of 30 psi was introduced into the system. During the test run the front glass cracked while leaving the back glass intact for over a month. A detailed analysis was then performed on the glass design. The pattern on the glass was uniform which proved that the flange design worked and pressures were evenly distributed. Unlike the quartz, the borosilicate during this run shattered instead of cracking in a straight line as previously shown. Figure 14: Cracks Pattern on Borosilicate Glass Page 26 of 51

27 Figure 15: Close-up of Cracks The crack patterns were consistent with that of borosilicate and the symmetry of the cracks proves that the flange design allowed for uniform loading. After further analysis it was discovered that glass under pressure is sensitive to 2 major characteristics: its thickness and finish. The 0.25 thick glass was within the calculated theoretical range with a safety factor of 2. However the new manufacturers glass cutting technique did not provide a smooth finish. This created sites for crack propagation and made the glass weaker. Due to this unfortunate flaw, the previous calculations were voided and the stress tolerance on the new glass lowered significantly. This caused the glass to break again forcing a further investigation. Glass Design Calculations An initial estimate of thickness required for the glass window was obtained from the relation below 6. This relation describes the required glass thickness for a window with clamped edged subjected to an internal pressure. Page 27 of 51

28 P= ( ) xm T AxF PxAxF T= 3.48 xm A is the unsupported Area in sq/inches, T is window thickness (inches), F is safety factor (7) M is modulus of rupture of Quartz (7000 psi), P is pressure (psi) An initial estimate of P = 1atm (14.7psi), for an unsupported area of 18in x 0.75in (as per established design) yielded: 14.7 x(18 x0.75) x7 T= =0.2388in 3.48x7000 Following the setbacks described earlier it was obvious that using the above equation alone was not enough to determine appropriate thickness of glass to be used. A highly detailed analysis was carried out using SolidWorks and CosmosWorks to determine glass thickness as well as maximum allowable torque. Computer Aided Analysis During the investigation two different techniques were used to check for the thickness of the glass and its ability to withstand a certain amount of stress. The first was described in the glass theory section above. A number of formulas were used to calculate the thickness of glass with relation to its clamping force/torque. The second technique was to use the values determined from the calculations and simulate them on CosmosWorks. This would determine stress patterns and allow further improvement of the chamber design. Page 28 of 51

29 We began our analysis by first considering the redesigned flange and ran simulations to see if the deformation was consistent with the crack patterns on the glass. We hypothesised that multiple shallow curves would arise in between the bolts providing a slightly uneven pressure gradient as shown in the figure below. Unlike the first design, the shallow curves would cause the glass to be in tension at spots between the bolts and not at the bolts. This coupled with the pre-existing cracks in the edges due to the bad finish caused the glass to eventually break. The figure below illustrates the deformation in the flange. The shape is exaggerated and the deformation is not as large as shown below. The scale irrelevant to this illustration because the most important thing to not is the shape of the loaded flange. Figure 16: Deformation in Design 2 of Flange To verify the validity of the techniques for glass, ¼ Borosilicate glass was used in the CosmosWorks model and the resulting stress patterns were analyzed to determine if they were similar to the crack patterns shown above. The pressures on the glass were applied according to the pressure gradient shown by the flange above. This was then replicated onto the glass to conduct the analysis. Page 29 of 51

30 Figure 17: Stress Analysis on 1/4" Pyrex (Borosilicate) As discussed in the glass section, the fracture strength of borosilicate is 1000psi, however once the finish is taken into account coupled with the effect of the buckling, stress at certain parts of the glass was observed to increase to approximately 6700 psi. Once the technique was verified the team used it to find a new dimension and material for glass that will not break at extreme conditions in the chamber. The pressures used were now in the 150 psi range and the torque up to 25ft.lbs. In addition, a recommended 7 FOS (Factor of Safety) was used during the simulation. This was applied to various thicknesses of glass and to borosilicate and quartz. After running a few simulations, a plate of Quartz glass of ¾ thickness was deemed ideal for these conditions and the stress and displacement plots depicted below were observed. Page 30 of 51

31 Figure 18: Stress Plot (Scale in Inches) As predicted, the stresses were well below the fracture stress of Quartz. A thicker piece also reduced buckling and a change in manufacturer improved the quality of finish of the glass. Page 31 of 51

32 3.2.3 Current Design This design is a modification of the redesigned flange design. The reason the previous design was modified was to accommodate a thicker piece of glass. After carrying out the in depth FEA and theoretical analysis described above it was decided that the properties of quartz better meets the requirements of this project than borosilicate. The decision to use a higher factor of safety led to a change of glass thickness from 0.25 to These modifications are illustrated in the 3D model of the chamber shown in the figure below. Figure 19: Exploded View - Current Design A new component was added to the flange to raise the depth of the seat for the glass. This was designed to press against the glass without creating alignment problems. Gaskets were added between the pieces to ensure a snug fit. The new thicker glass sat in the same place as previous designs and every other component was kept constant. The torque on the bolts was reduced to 20ft.lbs to avoid cracking or buckling. Page 32 of 51

33 Figure 20: Current Design 3.3 The Camera Assembly The bubbles produced due to nucleation are recorded using a high speed CCD camera, frame by frame using a frame grabber. The diameter, position and speed of bubbles and nucleation site density can be determined by this method. The camera set-up is highly complex and sensitive. The camera is set-up on a tripod right outside the chamber. Due to the high magnification of the camera being used, the tripod, although steady, is not the ideal mounting apparatus. A stage used in optics would be a great asset to this experiment. Due to lack of such a stage, a modified slider mechanism was used to allow for a 2 DOF system. This allowed the user to make small changes in the X and Y-axis. The ability to make slight changes makes it easier to focus the images required. The lighting used in this experiment also can be improved by replacing the current lighting with ones that have an option to vary light intensity and movement. Page 33 of 51

34 Figure 21: Camera Stage The right placement of the camera and lighting is extremely important to capture the fast moving bubbles in the chamber. It is also important to know where the bubbles will appear before the camera is set. Calculations: Point of Onset of Nucleation In order to get recordable results, it was important that nucleation occur within the 500mm length of the cartridge heater being used. One of the main constraints affecting the point of onset of nucleation is the power available from the cartridge heater. As stated earlier, for the required dimensions, a maximum wattage of 2500W was available in the market which resulted in a heat flux of approximately 80W/in 2 (1.27E+05W/m 2 ). The calculations shown in Table 1 were used to get a preliminary estimate of the point of onset of nucleate boiling for an initial design evaluation. Starting with conditions in actual nuclear reactor cores, iterations were performed by changing conditions such as flow velocity, inlet temperature and pressure and so on and their effects on nucleation point and mass flow rate were observed. After three iterations an acceptable nucleation point of Page 34 of 51

35 400mm was obtained using feasible values for pressure, temperature and heat flux. A sample calculation for the values obtained in the table below is shown in Appendix A. Iterations Initial Pressure (P) Bar 9.00E E E E+00 Inlet Temperature (T L,in ) K Saturation Temperature (T sat ) K Temperature Bulk Temperature (T B ) K Velocity (U L,in ) m/s Density (ρ L,in ) Kg/m Enthalpy (h fg ) kj/kg Thermal Conductivity (k L ) W/m*K Thermal Conductivity (k f ) W/m*K Density (ρ g ) Kg/m Density (ρ L ) Kg/m Viscosity (µ L ) kg/m*s 9.46E E E E-04 Fluid Properties Specific Heat (C pl ) J/kg*K Power (p) W 9.00E E E E+03 Diameter (D) m 1.25E E E E-02 Heater Dimensions Length (L) m 5.00E E E E-01 Diameter of Outer Tube (D o ) 2.00E E E E-02 Surface Tension (σ) N/m Page 35 of 51

36 Mass Flux (G) ρ L,in * U L,in Kg/m 2 *s Heat Flux (q") p/(π*d*l) W/m E E E E+05 Reynolds Number (Re L ) (G*D)/(µ L ) 9.58E E E E+03 Prandtl Number (Pr L ) (µ L *C p )/k 8.60E E E E+00 Nusselt Number (Nu D ) (H L0 *D)/k L Heat Transfer Coefficient (H L0 ) (0.023Re 0.8 L * Pr 0.4 L * k L )/D W/m 2 *K 6.06E E E E+02 Polytropic Exponent (n) 0.463*P E E E E-01 Wall Temperature (T w ) T sat *[q"/(1082P )] n K 6.06E E E E+02 Mass Flow Rate (m dot ) ρ L,in *(π/4)*(d 2 o -D 2 )*U L,in kg/s 1.39E E E E-02 Bulk Temperature at ONB (T L,ONB )* T w = T L,ONB K 6.06E E E E+02 Distance to Nucleation (Z ONB ) [m dot *C pl *(T L,ONB -T L,in )]/(π*d*q") m 1.06E E E E-01 * Since the distance between the walls are small, it was assumed that T w,onb = T L,ONB (i.e wall temperature = bulk liquid temperature) Variables that were changed during iteration Table 1 Nucleation Distance and Mass Flow Rate Calculations Page 36 of 51

37 In addition to these calculations it was decided to polish the heater surface to create a relatively smooth surface followed by small surface scratches to make nucleation sites. The camera was then focused directly at the scratch and images were captured. After the right resolution is achieved the heater is observed using the Photron Fastcam Viewer. For the purpose of this experiment, a scratch was made on the top side of the cartridge heater in three locations front, middle and end. This allowed the bubbles to travel upwards rather than travel outwards or hug the surface. To take the right image, it is important that the bubbles take off unobstructed in a predictable direction. After the bubble pictures were taken the reference scale placed inside the test area was used. This experiment used the size of the scratch as a reference. This picture will then be used to scale the bubble diameters and positions while the frame speeds will be used to determine bubble frequency. Page 37 of 51

38 4.0 Experimental Setup This section will include a short summary of how to begin the experiment and prepare the set-up. The set-up tends to take approximately 3 hrs before experimentation can begin. Figure 22: Schematics of the Entire Set-Up i. Fill up the tank. It is recommended that the tank be full so as to maximize the use of all the water. During this step, remember to leave valves closed except for the release valve. ii. Once full, close all valves and start the heater. During heating, keep the pump running to evenly heat the water. To start the heater, you must plug in the blue box Page 38 of 51

39 into the socket, turn the middle and left box on and use the left box to control and program desired temperature. Figure 23: From Left to Right - Data Acquisition System, Power Switch and Pump iii. Once the temperature has reached the desired inlet temperature, turn off the heater and pump, unplug the heater from the socket in the wall and plug in the cartridge heater (Same wall socket has to be used to satisfy power requirements for both the tank heater and cartridge heater). At this point do not turn on the cartridge heater. iv. Make sure all valves are close. Open the inlet valve for the compressed air both from the tank and the main building valve and pressurize the tank to approximately 100psi. This will help pump the water out of the tank. Note that the pump is only used for circulation and not for feeding water to the chamber. v. Now open the outlet valve located at the bottom of the chamber and let the water flow to the chambers ball valve. Page 39 of 51

40 vi. Make sure you do not open the ball valve until you are ready to take readings. Be careful of the hot exposed piping. vii. At this point make sure the camera has been installed at the desired location and focus, and the lighting is properly located. The camera procedure is just guidance and should only be used as a reference because the camera set-up may change every time. The key observation to keep in mind is that the camera needs to be set on a stage with 3 degrees of freedom X, Y and Z axis. This can help the user move the camera with excellent control. For future experiments a better lighting system is recommended. Lighting with mobility and variable illumination can make recording clearer and easier. Below is a part of the stage used with mobility in the x-direction. Figure 24: Example of camera stage Page 40 of 51

41 Figure 25: The Camera (Tripod Mounted) viii. Once the camera and software is set-up the outlet valve in the chamber must be opened to make sure that initially the internal pressure in the chamber is at atmospheric. ix. Now the ball valve in the inlet can be slowly opened to the desired flow rate. The outlet valve can be used to control the pressure in the chamber. x. The cartridge heater can be turned on now and controlled using another DAS and the right readings can be taken. xi. Once the experiment is over, the user must make sure that the inlet valve in the chamber is closed while the outlet is wide open. The tank must be completely drained and the pressure released from the tank. All valves on the tank should be fully opened to reduce the chances of any accident due to pressure build up. Note: Every time water flows through the chamber two things can happen. The image will either go blurry and bright due to the excessive amount of nucleation or it will go dark due to the Page 41 of 51

42 water. If the image goes blurry the user must increase either the water pressure or the mass flow to reduce the nucleation rate. This will allow only specific sites to nucleate. If the image goes dark the aperture on the lens can be used to brighten the image. The lighting can also be moved or intensified to attain the right contrast. The software also contains features that may help better capture the images. In this experiment, the use of frame rate and shutter speed will allow the video to grab as many pictures as possible. We used 1000 frames per second (fps) and a shutter speed of 1/frame. Page 42 of 51

43 5.0 Results and Discussion The setup developed can be used to obtain information about bubble departure diameter, bubble departure velocity and bubble frequency. The following subsections contain a description of the approach to proceed with this experiment. 5.1 General Recommendations Polishing the heater and making a scratch on the surface helps induce nucleation at that point. The rate of nucleation and number of nucleation sites increases with an increase in temperature of the inlet water and decrease in system pressure and flow rate. 5.2 Bubble Departure Diameter The development and evolution of bubbles can be observed by comparing data at consecutive time points. Extensive research indicated that Fritz model is one of the most reliable for predicting bubble diameter for pool boiling of pure liquids. The shape and form of the theoretical pressure versus bubble diameter curve obtained from this model would be compared to the graph obtained from actual measurements to determine if a new relation can be obtained by finding a correction factor that accurately fits the theoretical curve to the actual curve. A graph depicting effects of flow rate on diameter would also be added. Page 43 of 51

44 D g 2 l v Fritz Model 5.3 Bubble Velocity Stokes law is recommended for obtaining the theoretical curve and a comparison should be made with the actual curve to determine if it can be fitted by adding a correction factor. Graph of flow rate versus bubble velocity would also be added. V t 2 gd 18 l Stokes Law 5.4 Bubble Frequency Frequency can be determined by measuring the time lapse between the moment a bubble broke from the heater surface at a specific location to the moment the next bubble formed at the same location broke from the surface. Research did not yield any suitable theoretical relations and as a result it is recommended to create plots of pressure versus frequency and flow rate versus frequency. Page 44 of 51

45 6.0 Future Recommendations Throughout the life of this project there have been continuous modifications and improvements made to the design and testing procedure. Although these changes have been useful a set of recommendations to further improve the functionality of the set-up will be discussed in this chapter. The three major assemblies that require improvement are the chamber design and the tank and camera stage and lighting. For future experiments we would recommend using a stainless steel tank to improve the quality of the water coming into the chamber. Currently the use of a steel tank constantly feeds contaminated water that leads to a murky view through the glass. Also, all valves and fittings connected to the tank should also have the same material (stainless steel). This makes the tank cleaning process more effective and easier. Secondly, the chamber design can be modified to help accommodate the needs of the project. Aluminium, although reasonably priced, is not the best material to use when working with a glass window. The properties of aluminium cause it to warp under forces from the bolts. This causes uneven loading on the glass resulting in cracked glass. If the material was changed to stainless steel, the set-up would hold up better and the warping would be brought to a minimum. As mentioned in the design section, the manufacturers, quality of finish and glass types make a big difference on the strength of the glass. We recommend using Quartz of thickness ¾ with a clean finish (no visible cracks). Further Page 45 of 51

46 research into another type of glass called sapphire glass might produce a stronger, but more expensive alternative to Quartz. When installing the glass the user must torque the bolts uniformly and in a spiral motion as mentioned in the experimental set-up section. Figure 26: Spiral motion of tightening bolts (tighten bolts in this order) Lastly, major modifications are required in the camera set-up. The lack of a proper stage makes it difficult to make small displacements in the x, y and z axis. The stage designed for the experiment only moved in the x and y axis. A third axis can help find the exact site of nucleation without losing focus. The stage can also help with scaling. The size of the bubbles can be determined by using a reference in the chamber. Once the reference is used, the focus cannot be changed to maintain the scale. Having a sensitive stage can help maintain the the accuracy of the scale. In addition to the camera set-up, the lighting is one of the most important aspects of photography. The lack of proper fixtures, lighting and variability makes it very difficult to take a proper image. We recommend three sets of lighting fixtures two in the front from the left and right and one from the back. All three fixtures need to have adjustable luminosity and if possible, varying rates dispersion settings. If the camera can be mounted on a large stage, the lighting could also be mounted on the same stage. If the stage is small, the lighting would require a tripod or some other fixture that provides Page 46 of 51

47 the same degree of freedom as a tripod. Lastly, if the chamber was rebuilt, bigger window dimensions would be a definite recommendation. For the current set-up it is difficult to angle the camera because of the lack of visibility. A larger window would allow more lighting to penetrate giving the user more freedom to change the inclination angle of the camera. Page 47 of 51

48 7.0 Conclusion An experimental set-up was created to mimic a nuclear fuel channel for the purpose of studying the following characteristics of nucleation: bubble departure diameter, bubble departure velocity and bubble frequency. The design was considered successful after many changes were made. Towards the end of the project more insight for the design of the experimental set-up was understood. Some of the designed such as glass material and thickness were implemented. Others such as materials of the flange, tank and other modifications were documented as recommendations for the next set-up. The set-up was used by Lionel Lobo to study nucleation. A numerical model of the set-up is also being designed to further understand the characteristics of nucleation from a mathematical point of view. If the design set-up was rebuilt or improved, it is highly recommended to change the material used to build the chamber from aluminium to stainless steel to reduce warping when subject to high torque which could lead to uneven forces being applied to the glass window. Page 48 of 51

49 Appendix Iteration 1 Calculations Distance of NucleationError! Bookmark not defined. The density of water at inlet conditions is ρ L,in = 725kg/m 3. Therefore the mass flux is: G = ρ L,in U L,in = 725kg/m 3 x 1m/s = 725kg/m 2.s For average liquid properties we can approximate bulk temperature is 10K lower than wall temperature at the onset of nucleate boiling. In this case T B = 566K and therefore ρ L = 725kg/m 3, µ L = 9.46E-05kg/m.s, k L = 0.574W/m.k, C PL = 5220L/kg.k and Prandtl number Pr L = (µl*cp)/k = Re L = GD/µL = 9.58E+04 Figure 27: Schematics of the Entire Set-Up The correlation of Dittus and Boelter can be used to determine convective heat transfer coefficient, H L0 : H D Nu Re Pr H 9.61E 03 W / m. K L D L L L0 kl Polytropic exponent (n): n = 0.463P = Error! Reference source not found. T W,ONB = 606K The location of ONB is determined by performing the below energy balance: Page 49 of 51

50 Error! Reference source not found. Where m L = L, in Do D U L, in Substituting this value in Equation 13 gives: Z ONB = m = 10.6mm = 0.139kg/s Page 50 of 51

51 References [1] Joseph Gonyeau, The Virtual Nuclear Tourist, March 15, [Online]. Available: [Accessed: August 10, 2010] [2] R.S. Porter, Reactor Mechanical Design, October 1, [Online]. Available: [Accessed August 10, 2010] [3] Rayotek Inc., Choosing the Right material. [Online]. Available: [Accessed: February 20, 2010] [4] Holloway, The Physical Properties of Glass, Wykeham Publications, 1973 [5] Robert H. Doremus, Glass Science 2nd Edition, Wiley-Interscience, 1994 [6] Technical Glass Products Inc., Pressure Calculator. [Online]. Available: Page 51 of 51