- Email: [email protected]
Experimental study of onset of subcooled annular flow boiling
Progress in Nuclear Energy 51 (2009) 361–365
Contents lists available at ScienceDirect
Progress in Nuclear Energy journal homepage: www.elsevier.com/locate/pnucene
Experimental study of onset of subcooled annular flow boiling R. Ahmadi a, *, A. Nouri-Borujerdi a, J. Jafari b, I. Tabatabaei a a b
Department of Mechanical Engineering, Sharif University of Technology, Tehran, Iran Reactors and accelerators R&D School , Nuclear science and technology research institute Tehran , Iran
a b s t r a c t Keywords: Two-phase flow Experiments Onset of nucleate boiling Annular flow
An experimental study on the onset of nucleate boiling (ONB) is performed for water annular flow to provide a systematic database for low pressure and velocity conditions. A parametric study has been conducted to investigate the effect of pressure, inlet subcooling, heat and mass flux on flow boiling. The test section includes a Pyrex tube with 21 mm inner diameter and a stainless steel (SS-304) rod with outer diameter of 6 mm. Pressure, heat and mass flux are in the range of 1.73 < P < 3.82 bar, 40 < q < 450 kW/m2 and 70 < G < 620 kg/m2 s, respectively. The results illustrate that inception heat flux is extremely dependent on pressure, inlet subcooling temperature and mass flux; for example in pressure, velocity and inlet subcooling as 3.27 bar, 230 kg/m2 s and 41.3 C; consequently qw,ONB is 177.3 kW/ m2. In other case with higher inlet temperature of 71.5 C and with P, 3.13 bar and G, 232 kg/m2 s the inception heat flux reached to 101.6 kW/m2. The data of ONB heat flux are over estimated from the existing correlation, and maximum deviation of wall superheat (DTw,ONB) from correlations is 30%. Experimental data of inception heat flux are within 22% of that predicted from the correlation. Ó 2008 Published by Elsevier Ltd.
1. Introduction Partitioning the phase change during the subcooled flow boiling begins with onset of nucleate boiling (ONB). Upstream of the location of ONB, single phase heat transfer prevails and after it the bubbles are presented and different heat transfer mechanism comes in to play. Several scientists have studied subcooled flow boiling and particularly ONB. Hsu (1962) was the first to postulate the criteria for the boiling inception. He discussed that the bubbles are presented when the temperature at the tip of the bubble is at least equal to the saturation temperature corresponding to the pressure inside the bubble. The pressure difference between inside the bubble and liquid surrounding it can be expressed in terms of the Young–Laplace equation (Dp ¼ 2s/rb) for a spherical bubble. Hsu found the corresponding saturation inside the bubble by attending to Clausius–Clapeyron equation (Collier and Thome, 1996). The temperature profile in the thermal boundary layer can be assumed to drop linearly in the thermal boundary layer, and thermal boundary layer thickness is defined as dt ¼ kf/h. Stochastically averaged spherical or chopped-spherical bubbles, nevertheless, are central to the models for ONB (Bergles and Rohsenow, 1964; Sato and Matsumara, 1964; Davis and Anderson, 1966; Han and Griffith, 1965; Yin and Abdelmessih, 1974; Sudo et al., 1986; Marsh and Mudawar, 1989). Bergles and Rohsenow
* Corresponding author. E-mail address: [email protected] (R. Ahmadi). 0149-1970/$ – see front matter Ó 2008 Published by Elsevier Ltd. doi:10.1016/j.pnucene.2008.05.003
(1964) have expressed the equation for predicting ONB point in terms of excess wall temperature and operational pressure as, 0:0234
qw;ONB ¼ 1082p1:156 ½1:799ðTw Tsat Þ2:3=p
(1)
2
where P is in bar, qw,ONB is in W/m and T is in C. Their experiments were conducted with water on stainless steel (SS) and nickel surfaces and covered a range of pressures from 1.02 to 138 bar. Sato and Matsumara (1964) analytically studied the ONB phenomena in terms of wall superheat and based on Hsu criteria. They found the following expression:
qw;ONB ¼
kf hfg rv ðDTw Þ2 8sTsat
(2)
Davis and Anderson (1966) extended this analysis and introduced the contact angle as a variable for the ONB condition. Their analysis gave the heat flux condition at ONB as,
qw;ONB ¼
k1 hfg rv ðDTw Þ2 8C1 sTsat
(3)
where C1 ¼1 þ cos 4. Basu et al. (2002) studied ONB and active nucleation site density during subcooled flow boiling on flat plate and bundle of Zircalloy4 tube. Their study performs in one pressure and several ranges of flow rate and finds qw,ONB and DTw,ONB in several locations. Several experiments are studied with refrigerant fluid on different surfaces in low pressure (Basu et al., 2002). These experiments performed
362
R. Ahmadi et al. / Progress in Nuclear Energy 51 (2009) 361–365
Nomenclature D G h k p q r Re T v
diameter mass flux heat transfer coefficient thermal conductivity pressure heat flux radius Reynolds number rvDH/m temperature bulk velocity
Greek
D d r s m
difference boundary layer thickness density surface tension viscosity
with refrigerants and surfaces as R11/SS, Haynes-230; R12/Cu; R113/SS, Cr, Pt and FC-72/Ni. Recently Dong et al. (2005) and Situ et al. (2005, 2004) worked on the onset of nucleate boiling in microchannel flow and studied the bubble behaviors in forced convective subcooled boiling flow separately and found some new phenomenon at other cases. The objective of this study is to identify the parameters affecting ONB in subcooled flow boiling, compared to the experimental data with existing correlations. Several experiments were conducted covering a wide range of flow rates, pressure, liquid subcooling and heat fluxes on an SS rod. 2. Experimental apparatus Forced convection boiling on tubular wall is discussed in several cases, especially in vapor power plant study. In this case, treatment of water during heat transfer in vertical and annular geometry is discussed. The schematic of apparatus that provided for test loop is shown in Fig. 1. The flow loop consists of a stainless steel pressurizer, a linear pump, magnetic flow meter, bypass line, preheater and test section. There is in the pressurizer one immersion heater (5 kW total power) to degassing and preheat the water used in the experiments. The preheater consisted of a 5 kW (480 V, 3 phase) flanged immersion heater fitted vertically onto a stainless steel container. The power to the immersion heater is controlled using a thermostat. Using the power controller and thermocouple outputs, it is possible to control the liquid subcooling accurately. Thermocouples and pressure transducers are installed at the inlet and exit of the heating section. Three thermocouples are fitted on inner surface of SS pipe heater to detect the wall temperature and three thermocouples are mounted at the front of each to sense the fluid temperature. Required power for heater is supplied by coupled motor and generator. This equipment supplies 36-kW maximum total energy by producing 60 V (DC) and 600 A (DC). This power is sufficient for detect ONB point in that condition was discussed.
Subscripts b bubble c condensation ev evaporation fc forced convection g gas H hydraulic in inlet l liquid nb nucleate boiling ONB onset of nucleate boiling sat saturation sub subcooled t thickness w wall f fluid
current rating of the power supply. Stainless steel 304 has been chosen as the material heater and the cladding is joule effect heated using a 36 kW DC power supply. The diameter of sight glass is fixed (21 mm ID). Thermocouples are chosen K-type and detected the temperature of water in the overall test section from inlet to outlet and the temperature at the bottom, intermediate and top of heater wall. Pressure transducers are mounted in the inlet and outlet of water flow and activate with magnetic effect. Power to the tube is provided using copper bus bars mounted at the ends of the test section (at the inlet and exit of the heated section). Thus this configuration does not disturb the flow. The wall temperatures of the rod are measured at various axial locations using miniature thermocouples mounted inside the thin-walled tubes as shown in Fig. 1. The thermocouples are attached and covered with a nonconducting cover throughout except at the tips where they are covered with electrically insulated but thermally conducting material. Several thermocouples are mounted at various axial locations along the
3. Test section In this section we have a heated length of 1.1 m with a 0.73 m long active steel heater. Glass windows are provided on perimeter of the heater for visual observation. The diameter of the rod is chosen (6 mm OD) and the wall thickness is 0.5 mm. This value was chosen so that the maximum power could be generated for the
Fig. 1. Schematic of test loop.
R. Ahmadi et al. / Progress in Nuclear Energy 51 (2009) 361–365
363
Fig. 4. Wall temperature in term of heat flux in above of heater.
4. Experimental procedure
Fig. 2. Deviation of heat transfer coefficient during subcooled flow boiling.
flow channel to measure the liquid temperature profile. Thermocouples are placed at three axial locations starting at 14.5 cm from the start active length of heater thereafter at regular intervals of 20.4 cm. The average heat generation rate (and hence the heat flux) was calculated using the voltage and current supplied (power ¼ IV, I ¼ current, V ¼ voltage).
Several subcooled flow boiling experiments were performed while varying the flow rate, inlet subcooling and pressure inlet with water. Prior to each experiment, pressurizer was filled with water and was heated. The water was degassed by boiling it for approximately 3 h and thereafter it was cooled to the required temperature. During an experiment, water was pumped from pressurizer through the flow meter, inline preheater, and the test section. The liquid flow rate was controlled using the valves placed at the preheater inlet and the bypass line. The power to the boiling surface was turned on once the required flow rate and liquid subcooling levels at inlet were achieved. After the test heater reached steady state, all the required temperature measurements were taken. A 16-bit data acquisition system (Model TCL-1810 that can attach to slat TCI) was used to record the temperatures, pressure, voltage and ampere. 5. Result and conclusion In this work a mechanistic model has been developed for the prediction of onset of nucleate boiling wall heat flux, as a function of wall superheat, for subcooled flow boiling. The basic difference of
Fig. 3. Heat transfer coefficient vs. heat flux at three locations of heat surface.
Fig. 5. Comparison of predicted an experimental 6Tw, ONB values.
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R. Ahmadi et al. / Progress in Nuclear Energy 51 (2009) 361–365
Fig. 8. Inception heat flux in several mass flux.
this model from the existing ones is that in this model a bottom-up approach is used and it is proposed that all the wall energy is first transferred to the superheated liquid layer adjacent to the wall, from where a fraction of it goes to vaporization. Also note that from this model it is possible to predict the DTw as a function of an applied q for a given geometry and flow conditions. The experiment covered a range of pressure, P, varying from 1.73 to 3.82 bar, mass fluxes, G, varying from 70 to 620 kg/m2 s, DTsub,in from 39 to 101 C and qw from 40 to 450 kW/m2. The onset of nucleate boiling is a key parameter in the design of thermo hydraulic systems and in some cases the first limiting parameter against heat flux. The onset of nucleate boiling is the location where the first vapor bubbles appear on the heater surface. Determination of this point was performed by visual observation method and by magnifier 9. The accuracy of this method can be validated by determining the location of deviation of heat flux coefficient during axial flow boiling. Fig. 2 shows the h–Z/L plots for two cases of experiment condition. In one case, conditions were set as G ¼ 156 kg/m2 s, P ¼ 1.83 bar and DTsub ¼ 76.1 C, and heat flux (qw) was varied from single phase (qw ¼ 8.39 kW/cm2) to complete two -phase flow (qw ¼ 17.2 kW/cm2) in overall test section. In this figure predicted ONB point is shown by small vertical line in deviation curve of coefficient heat transfer. ONB points are observed near these lines.
Power is increased continually until the total heat section becomes two-phase flow regime. Fig. 3 shows the coefficient heat transfer in terms of wall heat flux. The ONB location of three points on the heater along annular test section are shown in this figure. In other case, pressure and flow rate consequently 3.81 bar and 470 kg/m2K with inlet temperature as 42 C are set and deviation of heat transfer coefficient in several powers is plotted in Fig. 2. In this case, location of observation of ONB is denoted with small vertical line and following figures identify ONB heat flux in three points of test section. By increasing the surface heat flux the location of ONB is moved downward of test section until at 411.4 kW/m2 overall of heater is covered with bubble or in other words whole of test section is in two-phase flow boiling. Other method for recognizing the ONB point is finding location of varying the rate of wall temperature during increasing heat flux. This location is single phase and two-phase flow boundary. For the above condition that was discussed (i.e. P ¼ 3.81 bar, G ¼ 470 kg/ m2 s, Tin ¼ 42 C) ONB wall temperature per inlet subcooled temperature (DTw,ONB/DTsub,in) in terms of heat flux is shown in Fig. 4. During single phase subcooled flow boiling in test section, temperature is varying linearly and after the first bubble is produced rating of the varying temperature is decreased. Location of ONB is stated above horizontal axes. In this case study of subcooled flow boiling, all data that were gathered compared with accessible correlations. Figs. 5 and 6 show that this study of subcooled flow boiling model predict DTw,ONB
Fig. 7. Inception heat flux in several pressure.
Fig. 9. Inception heat flux vs. inlet temperature.
Fig. 6. Comparison of predicted an experimental qw, ONB values.
R. Ahmadi et al. / Progress in Nuclear Energy 51 (2009) 361–365
within 30% and qw,ONB within 22% for a wide range of pressure, flow rate, inlet liquid subcooling. These figures compared with Bergles and Rohsenow’s correlation. In Figs. 7–9 effect of pressure, mass flux and inlet subcooling is shown. 6. Summary In this study ONB location, excess super heating wall temperature and heat flux in annular subcooled flow boiling were discussed and validity of the ONB correlation has been shown. Experiments are performed in low pressure and low flow rate. Reynolds numbers are within 1961–15,688, so the flow is turbulent. In this experimental study the ONB heat flux during the pressure and mass flux within 1.73 < P < 3.82 bar and 70 < G < 620 kg/m2 s, respectively, is varied from 40 to 450 kW/m2. References Basu, N., Warrier, G.R., Dhir, V.K., 2002. Onset of nucleate boiling and active nucleate boiling and active nucleation site density during subcooled flow boiling. ASME J. Heat Transf. 124, 717–728.
365
Bergles, A.E., Rohsenow, W.M., 1964. The determination of forced-convection surface-boiling heat transfer. ASME J. Heat Transf. 1, 365–372. Collier, J.G., Thome, J.R., 1996. Convective Boiling and Condensation, third ed. In: Oxford Engineering Science Series OUP. Davis, E.J., Anderson, G.H., 1966. The incipience of nucleate boiling in forced convection flow. AIChE J. 12 (4), 774–780. Dong, L., Lee, Poh-Seng, Suresh, V.G., 2005. Prediction of the onset of nucleate boiling in microchannel flow. Int. J. Heat Mass Transf. 48, 5134–5149. Han, C.Y., Griffith, P., 1965. The mechanism of heat transfer in nucleate pool boiling – part I. Int. J. Heat Mass Transf. 8, 887–904. Hsu, Y.Y., 1962. On the size range of active nucleation cavities on a heating surface. ASME J. Heat Transf. 84, 207–216. Marsh, W.J., Mudawar, I., 1989. Predicting the onset of nucleate boiling in wavy freefalling turbulent liquid films. Int. J. Heat Mass Transf. 32 (2), 361–378. Sato, T., Matsumara, H., 1964. On the conditions of incipient subcooled boiling with forced convection. Bull. JSME 7 (26), 392–398. Situ, Rong, Hibiki, Takashi, Ishii, Mamoru, Mori, Michitsugu, 2005. Bubble lift-off size in forced convective subcooled boiling flow. Int. J. Heat Mass Transf. Situ, Rong, Mi, Ye, Ishii, Mamoru, Mori, Michitsugu, 2004. Photographic study of bubble behaviors in forced convection subcooled boiling. Int. J. Heat Mass Transf. Sudo, Y., Miyata, K., Ikawa, H., Kaminaga, M., 1986. Experimental study of incipient nucleate boiling in narrow vertical rectangular channel simulating sub-channel of upgraded JRR-3. J. Nucl. Sci. Technol. 23, 73–82. Yin, S.T., Abdelmessih, A.H., 1974. Measurements of Liquid Superheat, Hysteresis Effects and Incipient Boiling Oscillations of Freon-11 in Forced Convection Vertical Flow. University of Toronto. Mech. Eng. Tech. Pub. Ser., TP-7401.
Contents lists available at ScienceDirect
Progress in Nuclear Energy journal homepage: www.elsevier.com/locate/pnucene
Experimental study of onset of subcooled annular flow boiling R. Ahmadi a, *, A. Nouri-Borujerdi a, J. Jafari b, I. Tabatabaei a a b
Department of Mechanical Engineering, Sharif University of Technology, Tehran, Iran Reactors and accelerators R&D School , Nuclear science and technology research institute Tehran , Iran
a b s t r a c t Keywords: Two-phase flow Experiments Onset of nucleate boiling Annular flow
An experimental study on the onset of nucleate boiling (ONB) is performed for water annular flow to provide a systematic database for low pressure and velocity conditions. A parametric study has been conducted to investigate the effect of pressure, inlet subcooling, heat and mass flux on flow boiling. The test section includes a Pyrex tube with 21 mm inner diameter and a stainless steel (SS-304) rod with outer diameter of 6 mm. Pressure, heat and mass flux are in the range of 1.73 < P < 3.82 bar, 40 < q < 450 kW/m2 and 70 < G < 620 kg/m2 s, respectively. The results illustrate that inception heat flux is extremely dependent on pressure, inlet subcooling temperature and mass flux; for example in pressure, velocity and inlet subcooling as 3.27 bar, 230 kg/m2 s and 41.3 C; consequently qw,ONB is 177.3 kW/ m2. In other case with higher inlet temperature of 71.5 C and with P, 3.13 bar and G, 232 kg/m2 s the inception heat flux reached to 101.6 kW/m2. The data of ONB heat flux are over estimated from the existing correlation, and maximum deviation of wall superheat (DTw,ONB) from correlations is 30%. Experimental data of inception heat flux are within 22% of that predicted from the correlation. Ó 2008 Published by Elsevier Ltd.
1. Introduction Partitioning the phase change during the subcooled flow boiling begins with onset of nucleate boiling (ONB). Upstream of the location of ONB, single phase heat transfer prevails and after it the bubbles are presented and different heat transfer mechanism comes in to play. Several scientists have studied subcooled flow boiling and particularly ONB. Hsu (1962) was the first to postulate the criteria for the boiling inception. He discussed that the bubbles are presented when the temperature at the tip of the bubble is at least equal to the saturation temperature corresponding to the pressure inside the bubble. The pressure difference between inside the bubble and liquid surrounding it can be expressed in terms of the Young–Laplace equation (Dp ¼ 2s/rb) for a spherical bubble. Hsu found the corresponding saturation inside the bubble by attending to Clausius–Clapeyron equation (Collier and Thome, 1996). The temperature profile in the thermal boundary layer can be assumed to drop linearly in the thermal boundary layer, and thermal boundary layer thickness is defined as dt ¼ kf/h. Stochastically averaged spherical or chopped-spherical bubbles, nevertheless, are central to the models for ONB (Bergles and Rohsenow, 1964; Sato and Matsumara, 1964; Davis and Anderson, 1966; Han and Griffith, 1965; Yin and Abdelmessih, 1974; Sudo et al., 1986; Marsh and Mudawar, 1989). Bergles and Rohsenow
* Corresponding author. E-mail address: [email protected] (R. Ahmadi). 0149-1970/$ – see front matter Ó 2008 Published by Elsevier Ltd. doi:10.1016/j.pnucene.2008.05.003
(1964) have expressed the equation for predicting ONB point in terms of excess wall temperature and operational pressure as, 0:0234
qw;ONB ¼ 1082p1:156 ½1:799ðTw Tsat Þ2:3=p
(1)
2
where P is in bar, qw,ONB is in W/m and T is in C. Their experiments were conducted with water on stainless steel (SS) and nickel surfaces and covered a range of pressures from 1.02 to 138 bar. Sato and Matsumara (1964) analytically studied the ONB phenomena in terms of wall superheat and based on Hsu criteria. They found the following expression:
qw;ONB ¼
kf hfg rv ðDTw Þ2 8sTsat
(2)
Davis and Anderson (1966) extended this analysis and introduced the contact angle as a variable for the ONB condition. Their analysis gave the heat flux condition at ONB as,
qw;ONB ¼
k1 hfg rv ðDTw Þ2 8C1 sTsat
(3)
where C1 ¼1 þ cos 4. Basu et al. (2002) studied ONB and active nucleation site density during subcooled flow boiling on flat plate and bundle of Zircalloy4 tube. Their study performs in one pressure and several ranges of flow rate and finds qw,ONB and DTw,ONB in several locations. Several experiments are studied with refrigerant fluid on different surfaces in low pressure (Basu et al., 2002). These experiments performed
362
R. Ahmadi et al. / Progress in Nuclear Energy 51 (2009) 361–365
Nomenclature D G h k p q r Re T v
diameter mass flux heat transfer coefficient thermal conductivity pressure heat flux radius Reynolds number rvDH/m temperature bulk velocity
Greek
D d r s m
difference boundary layer thickness density surface tension viscosity
with refrigerants and surfaces as R11/SS, Haynes-230; R12/Cu; R113/SS, Cr, Pt and FC-72/Ni. Recently Dong et al. (2005) and Situ et al. (2005, 2004) worked on the onset of nucleate boiling in microchannel flow and studied the bubble behaviors in forced convective subcooled boiling flow separately and found some new phenomenon at other cases. The objective of this study is to identify the parameters affecting ONB in subcooled flow boiling, compared to the experimental data with existing correlations. Several experiments were conducted covering a wide range of flow rates, pressure, liquid subcooling and heat fluxes on an SS rod. 2. Experimental apparatus Forced convection boiling on tubular wall is discussed in several cases, especially in vapor power plant study. In this case, treatment of water during heat transfer in vertical and annular geometry is discussed. The schematic of apparatus that provided for test loop is shown in Fig. 1. The flow loop consists of a stainless steel pressurizer, a linear pump, magnetic flow meter, bypass line, preheater and test section. There is in the pressurizer one immersion heater (5 kW total power) to degassing and preheat the water used in the experiments. The preheater consisted of a 5 kW (480 V, 3 phase) flanged immersion heater fitted vertically onto a stainless steel container. The power to the immersion heater is controlled using a thermostat. Using the power controller and thermocouple outputs, it is possible to control the liquid subcooling accurately. Thermocouples and pressure transducers are installed at the inlet and exit of the heating section. Three thermocouples are fitted on inner surface of SS pipe heater to detect the wall temperature and three thermocouples are mounted at the front of each to sense the fluid temperature. Required power for heater is supplied by coupled motor and generator. This equipment supplies 36-kW maximum total energy by producing 60 V (DC) and 600 A (DC). This power is sufficient for detect ONB point in that condition was discussed.
Subscripts b bubble c condensation ev evaporation fc forced convection g gas H hydraulic in inlet l liquid nb nucleate boiling ONB onset of nucleate boiling sat saturation sub subcooled t thickness w wall f fluid
current rating of the power supply. Stainless steel 304 has been chosen as the material heater and the cladding is joule effect heated using a 36 kW DC power supply. The diameter of sight glass is fixed (21 mm ID). Thermocouples are chosen K-type and detected the temperature of water in the overall test section from inlet to outlet and the temperature at the bottom, intermediate and top of heater wall. Pressure transducers are mounted in the inlet and outlet of water flow and activate with magnetic effect. Power to the tube is provided using copper bus bars mounted at the ends of the test section (at the inlet and exit of the heated section). Thus this configuration does not disturb the flow. The wall temperatures of the rod are measured at various axial locations using miniature thermocouples mounted inside the thin-walled tubes as shown in Fig. 1. The thermocouples are attached and covered with a nonconducting cover throughout except at the tips where they are covered with electrically insulated but thermally conducting material. Several thermocouples are mounted at various axial locations along the
3. Test section In this section we have a heated length of 1.1 m with a 0.73 m long active steel heater. Glass windows are provided on perimeter of the heater for visual observation. The diameter of the rod is chosen (6 mm OD) and the wall thickness is 0.5 mm. This value was chosen so that the maximum power could be generated for the
Fig. 1. Schematic of test loop.
R. Ahmadi et al. / Progress in Nuclear Energy 51 (2009) 361–365
363
Fig. 4. Wall temperature in term of heat flux in above of heater.
4. Experimental procedure
Fig. 2. Deviation of heat transfer coefficient during subcooled flow boiling.
flow channel to measure the liquid temperature profile. Thermocouples are placed at three axial locations starting at 14.5 cm from the start active length of heater thereafter at regular intervals of 20.4 cm. The average heat generation rate (and hence the heat flux) was calculated using the voltage and current supplied (power ¼ IV, I ¼ current, V ¼ voltage).
Several subcooled flow boiling experiments were performed while varying the flow rate, inlet subcooling and pressure inlet with water. Prior to each experiment, pressurizer was filled with water and was heated. The water was degassed by boiling it for approximately 3 h and thereafter it was cooled to the required temperature. During an experiment, water was pumped from pressurizer through the flow meter, inline preheater, and the test section. The liquid flow rate was controlled using the valves placed at the preheater inlet and the bypass line. The power to the boiling surface was turned on once the required flow rate and liquid subcooling levels at inlet were achieved. After the test heater reached steady state, all the required temperature measurements were taken. A 16-bit data acquisition system (Model TCL-1810 that can attach to slat TCI) was used to record the temperatures, pressure, voltage and ampere. 5. Result and conclusion In this work a mechanistic model has been developed for the prediction of onset of nucleate boiling wall heat flux, as a function of wall superheat, for subcooled flow boiling. The basic difference of
Fig. 3. Heat transfer coefficient vs. heat flux at three locations of heat surface.
Fig. 5. Comparison of predicted an experimental 6Tw, ONB values.
364
R. Ahmadi et al. / Progress in Nuclear Energy 51 (2009) 361–365
Fig. 8. Inception heat flux in several mass flux.
this model from the existing ones is that in this model a bottom-up approach is used and it is proposed that all the wall energy is first transferred to the superheated liquid layer adjacent to the wall, from where a fraction of it goes to vaporization. Also note that from this model it is possible to predict the DTw as a function of an applied q for a given geometry and flow conditions. The experiment covered a range of pressure, P, varying from 1.73 to 3.82 bar, mass fluxes, G, varying from 70 to 620 kg/m2 s, DTsub,in from 39 to 101 C and qw from 40 to 450 kW/m2. The onset of nucleate boiling is a key parameter in the design of thermo hydraulic systems and in some cases the first limiting parameter against heat flux. The onset of nucleate boiling is the location where the first vapor bubbles appear on the heater surface. Determination of this point was performed by visual observation method and by magnifier 9. The accuracy of this method can be validated by determining the location of deviation of heat flux coefficient during axial flow boiling. Fig. 2 shows the h–Z/L plots for two cases of experiment condition. In one case, conditions were set as G ¼ 156 kg/m2 s, P ¼ 1.83 bar and DTsub ¼ 76.1 C, and heat flux (qw) was varied from single phase (qw ¼ 8.39 kW/cm2) to complete two -phase flow (qw ¼ 17.2 kW/cm2) in overall test section. In this figure predicted ONB point is shown by small vertical line in deviation curve of coefficient heat transfer. ONB points are observed near these lines.
Power is increased continually until the total heat section becomes two-phase flow regime. Fig. 3 shows the coefficient heat transfer in terms of wall heat flux. The ONB location of three points on the heater along annular test section are shown in this figure. In other case, pressure and flow rate consequently 3.81 bar and 470 kg/m2K with inlet temperature as 42 C are set and deviation of heat transfer coefficient in several powers is plotted in Fig. 2. In this case, location of observation of ONB is denoted with small vertical line and following figures identify ONB heat flux in three points of test section. By increasing the surface heat flux the location of ONB is moved downward of test section until at 411.4 kW/m2 overall of heater is covered with bubble or in other words whole of test section is in two-phase flow boiling. Other method for recognizing the ONB point is finding location of varying the rate of wall temperature during increasing heat flux. This location is single phase and two-phase flow boundary. For the above condition that was discussed (i.e. P ¼ 3.81 bar, G ¼ 470 kg/ m2 s, Tin ¼ 42 C) ONB wall temperature per inlet subcooled temperature (DTw,ONB/DTsub,in) in terms of heat flux is shown in Fig. 4. During single phase subcooled flow boiling in test section, temperature is varying linearly and after the first bubble is produced rating of the varying temperature is decreased. Location of ONB is stated above horizontal axes. In this case study of subcooled flow boiling, all data that were gathered compared with accessible correlations. Figs. 5 and 6 show that this study of subcooled flow boiling model predict DTw,ONB
Fig. 7. Inception heat flux in several pressure.
Fig. 9. Inception heat flux vs. inlet temperature.
Fig. 6. Comparison of predicted an experimental qw, ONB values.
R. Ahmadi et al. / Progress in Nuclear Energy 51 (2009) 361–365
within 30% and qw,ONB within 22% for a wide range of pressure, flow rate, inlet liquid subcooling. These figures compared with Bergles and Rohsenow’s correlation. In Figs. 7–9 effect of pressure, mass flux and inlet subcooling is shown. 6. Summary In this study ONB location, excess super heating wall temperature and heat flux in annular subcooled flow boiling were discussed and validity of the ONB correlation has been shown. Experiments are performed in low pressure and low flow rate. Reynolds numbers are within 1961–15,688, so the flow is turbulent. In this experimental study the ONB heat flux during the pressure and mass flux within 1.73 < P < 3.82 bar and 70 < G < 620 kg/m2 s, respectively, is varied from 40 to 450 kW/m2. References Basu, N., Warrier, G.R., Dhir, V.K., 2002. Onset of nucleate boiling and active nucleate boiling and active nucleation site density during subcooled flow boiling. ASME J. Heat Transf. 124, 717–728.
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Bergles, A.E., Rohsenow, W.M., 1964. The determination of forced-convection surface-boiling heat transfer. ASME J. Heat Transf. 1, 365–372. Collier, J.G., Thome, J.R., 1996. Convective Boiling and Condensation, third ed. In: Oxford Engineering Science Series OUP. Davis, E.J., Anderson, G.H., 1966. The incipience of nucleate boiling in forced convection flow. AIChE J. 12 (4), 774–780. Dong, L., Lee, Poh-Seng, Suresh, V.G., 2005. Prediction of the onset of nucleate boiling in microchannel flow. Int. J. Heat Mass Transf. 48, 5134–5149. Han, C.Y., Griffith, P., 1965. The mechanism of heat transfer in nucleate pool boiling – part I. Int. J. Heat Mass Transf. 8, 887–904. Hsu, Y.Y., 1962. On the size range of active nucleation cavities on a heating surface. ASME J. Heat Transf. 84, 207–216. Marsh, W.J., Mudawar, I., 1989. Predicting the onset of nucleate boiling in wavy freefalling turbulent liquid films. Int. J. Heat Mass Transf. 32 (2), 361–378. Sato, T., Matsumara, H., 1964. On the conditions of incipient subcooled boiling with forced convection. Bull. JSME 7 (26), 392–398. Situ, Rong, Hibiki, Takashi, Ishii, Mamoru, Mori, Michitsugu, 2005. Bubble lift-off size in forced convective subcooled boiling flow. Int. J. Heat Mass Transf. Situ, Rong, Mi, Ye, Ishii, Mamoru, Mori, Michitsugu, 2004. Photographic study of bubble behaviors in forced convection subcooled boiling. Int. J. Heat Mass Transf. Sudo, Y., Miyata, K., Ikawa, H., Kaminaga, M., 1986. Experimental study of incipient nucleate boiling in narrow vertical rectangular channel simulating sub-channel of upgraded JRR-3. J. Nucl. Sci. Technol. 23, 73–82. Yin, S.T., Abdelmessih, A.H., 1974. Measurements of Liquid Superheat, Hysteresis Effects and Incipient Boiling Oscillations of Freon-11 in Forced Convection Vertical Flow. University of Toronto. Mech. Eng. Tech. Pub. Ser., TP-7401.