Browsing by Author "Lin, TF"

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A combined numerical and experimental study of air convection in a differentially heated rotating cubic cavity
(1996-10-01)
Ker, YT
;
Lin, TF
;
機械工程學系
;
Department of Mechanical Engineering
A combined numerical and experimental study was carried out to investigate the steady convective flow structure and the how stability in an inclined cubic air cavity subject to differential heating across a pair of side walls and subject to rotation about an axis which is normal to the insulated bottom wall and through the geometric center of the cavity. In the numerical simulation the three-dimensional Navier-Stokes and energy equations were directly solved by the projection method for the vertical cavity (phi = 0 degrees). While in the experimental study the air temperature in the cavity at selected locations was detected for both steady and unsteady flows. Results were obtained for Delta T = 10 degrees C and 4.6 degrees C, Omega from 0 rpm to 453 rpm and phi from 0 degrees to 75 degrees. The numerical results clearly revealed the complex multicellular flow structure in a vertical cavity at low rotation rates in which the mutual interactions of the thermal buoyancy, Coriolis force and rotational buoyancy are important. The experimental data suggested that the air flow can be stabilized by the cavity rotation when the rotation rate is low. But al high rotating speed the cavity rotation produced destabilizing effects. Copyright (C) 1996 Elsevier Science Ltd.
Buoyancy driven vortex flow and thermal structures in a very low Reynolds number mixed convective gas flow through a horizontal channel
(2003-06-01)
Lin, TF
;
機械工程學系
;
Department of Mechanical Engineering
This paper starts with an overview of the buoyancy driven vortex flow and the associated thermal characteristics in low Reynolds number mixed convection of gas through a bottom heated horizontal plane channel. Various vortex flow patterns reported in the past decades from theoretical, experimental and numerical explorations are examined, including the longitudinal, transverse and mixed vortex rolls. The mixed vortex rolls exist in several forms and result from the merging and/or splitting of the longitudinal and transverse rolls which are simultaneously present in certain parameter ranges. Then, the buoyancy induced formation of the longitudinal and transverse vortex flows from the unidirectional forced convection dominated flow in the channel is illustrated. Besides, the vortex flow affected by the aspect ratio of the channel is also discussed. Moreover, the stabilization of the unstable vortex flow driven at very high buoyancy-to-inertia ratios through the top plate inclination, top plate heating and a surface-mounted rib is inspected. Furthermore, our recent results for the mixed convective vortex flow structures over a heated circular substrate are presented. Finally, directions for future research are pointed out according to the need for thin film vapor phase epitaxy frequently used in microelectronic fabrication. (C) 2003 Elsevier Science Inc. All rights reserved.
Changes of longitudinal vortex roll structure in a mixed convective air flow through a horizontal plane channel: An experimental study
(1997-01-01)
Chang, MY
;
Yu, CH
;
Lin, TF
;
機械工程學系
;
Department of Mechanical Engineering
Flow visualization and temperature measurement were conducted to investigate the buoyancy induced longitudinal vortex how structure in a mixed convective air flow through a bottom heated horizontal plane channel for the Reynolds number ranging from 20 to 50 and Rayleigh number From 6000 io 30 000. The results suggested that at low buoyancy-to-inertia ratio a longitudinal roll pair with spanwise symmetry is first induced near the duct sides and more rolls are induced adjacent to the existing rolls as the flow moves downstream. Steady vortex flow prevails after the initial transient. At high buoyancy-to-inertia ratio the splitting and merging of the rolls take place frequently causing the flow to become unsteady and highly asymmetric at high Rayleigh numbers. Several types of the roll splitting processes were identified, namely, the splitting of one roll to three rolls and two rolls to four rolls, formation of two new rolls and two successive roll splittings of the above types. The reverse processes were noted during the roll merging. The data from the measured time histories of the air temperature indicated that the flow oscillation is position dependent with the oscillation amplitude being higher in the downstream core regions. Besides, the oscillation amplitude inside a roll is determined largely by the spanwise or axial position adjustment during the roll splitting and merging processes. Finally, the data for the onset of steady and unsteady vortex flows were empirically correlated. Copyright (C) 1996 Elsevier Science Ltd.
Characteristics of vortex flow in a low speed air jet impinging onto a heated disk in a vertical cylindrical chamber
(2003-11-01)
Hsieh, JC
;
Cheng, TC
;
Lin, TF
;
機械工程學系
;
Department of Mechanical Engineering
An experiment combining flow visualization and transient temperature measurement is carried out to investigate the characteristics of the mixed convective vortex flow resulting from a low speed air jet impinging onto a heated horizontal circular disk confined in a vertical adiabatic cylindrical chamber. Attention is focused on the conditions leading to the onset of the inertia and buoyancy driven vortex rolls and the effects of governing nondimensional groups on the steady and time dependent vortex flow. More specifically, experiments are conducted for the jet Reynolds number varied from 0 to 1082 and Rayleigh number from 0 to 18,790 for two different injection pipes. The results show that typically the steady vortex flow in the processing chamber consists of two inertia-driven and one buoyancy-driven circular vortex rolls. The secondary inertia-driven roll only appears at high jet Reynolds numbers. At low buoyancy-to-inertia ratio Gr/Re-j(2) the vortex rolls are steady and axisymmetric. But at certain high Gr/Re-j(2) the vortex flow becomes unstable and the vortex rolls are somewhat deformed. Besides, new vortex rolls can be induced by the additional thermal rising from the heated disk and the splitting of the primary inertia-driven roll. The temporal characteristics of the time periodic vortex flows are examined in detail. In the region dominated by the new rolls the flow oscillates significantly. Finally, empirical equations are proposed to correlate the oscillation frequency of the time periodic flow, and the size and location of the vortex rolls. Furthermore, the conditions for the onset of the buoyancy driven rolls are given. A flow regime map is provided to delineate the temporal state of the vortex flow. (C) 2003 Elsevier Ltd. All rights reserved.
Computation of unstable liquid metal convection in a vertical closed cylinder heated from the side and cooled from above
(1997-08-29)
Li, YH
;
Lin, KC
;
Lin, TF
;
機械工程學系
;
Department of Mechanical Engineering
Three-dimensional unsteady numerical computation was carried out to investigate buoyancy-driven unstable convection processes in a side-wad-heated and top-wad-cooled vertical closed cylinder containing liquid metal. The time-dependent basic flow equations were directly solved by the implicit Marker-and-Cell (MAC) scheme with the power law approximation to treat the convection-diffusion couplings. Results were obtained for liquid metal (Pr = 0.02) in a cylinder of finite aspect ratio (Gamma = 2) for various Rayleigh numbers. The predicted results indicate that the flow is relatively unstable except at low buoyancy. Specifically, of Ra = 2000 the flow evolves to a steady state but is highly asymmetric. As Ra = 3000 and 4000, the flow Becomes time periodic after the transient has died out with increasing asymmetry. At a still higher buoyancy the oscillation amplitude and flow asymmetry get larger, and a second fundamental frequency appears. The flow is quasi-periodic. Qualitatively, the flow structure is characterized by a main recirculation ill the major portion of the cylinder along with two uprising smaller cells near the side wall in the bottom half of the cylinder and another two counter rotating cells in the top half of the cylinder Finally, the critical Rayleigh number for the transition from a steady to a time periodic state was evaluated.
Computation of unstable liquid metal convection in a vertical closed cylinder heated from the side and cooled from above
(1997-08-29)
Li, YH
;
Lin, KC
;
Lin, TF
;
機械工程學系
;
Department of Mechanical Engineering
Three-dimensional unsteady numerical computation was carried out to investigate buoyancy-driven unstable convection processes in a side-wad-heated and top-wad-cooled vertical closed cylinder containing liquid metal. The time-dependent basic flow equations were directly solved by the implicit Marker-and-Cell (MAC) scheme with the power law approximation to treat the convection-diffusion couplings. Results were obtained for liquid metal (Pr = 0.02) in a cylinder of finite aspect ratio (Gamma = 2) for various Rayleigh numbers. The predicted results indicate that the flow is relatively unstable except at low buoyancy. Specifically, of Ra = 2000 the flow evolves to a steady state but is highly asymmetric. As Ra = 3000 and 4000, the flow Becomes time periodic after the transient has died out with increasing asymmetry. At a still higher buoyancy the oscillation amplitude and flow asymmetry get larger, and a second fundamental frequency appears. The flow is quasi-periodic. Qualitatively, the flow structure is characterized by a main recirculation ill the major portion of the cylinder along with two uprising smaller cells near the side wall in the bottom half of the cylinder and another two counter rotating cells in the top half of the cylinder Finally, the critical Rayleigh number for the transition from a steady to a time periodic state was evaluated.
Condensation heat transfer and pressure drop of refrigerant R-134a in a plate heat exchanger
(1999-03-01)
Yan, YY
;
Lio, HC
;
Lin, TF
;
機械工程學系
;
Department of Mechanical Engineering
An experimental refrigerant loop has been established in the present study to measure the condensation heat transfer coefficient h(r) and frictional pressure drop Delta P-f of R-134a in a vertical plate heat exchanger. Two vertical counter flow channels were formed in the exchanger by three plates of commercialized geometry with a corrugated sinusoidal shape of a chevron angle of 60 degrees. Downflow of the condensing R-134a in one channel releases heat to the cold upflow of water in the other channel. The effects of the refrigerant mass flux, average imposed heat flux, system pressure (saturated temperature) and vapor quality of R-134a on the measured data were explored in detail. The results indicate that at a higher vapor quality the condensation heat transfer coefficient and pressure drop are significantly higher. A rise in the refrigerant mass flux only causes a mild increase in the it, values h(r) most cases. The corresponding rise in the Delta P-f value is slightly larger. Furthermore, it is noted that the condensation heat transfer is only slightly better for a higher average imposed heat flux. But the associated rise in Delta P-f is larger. Finally? at a higher system pressure the h(r) value is found to be slightly lower. But the effect of the system pressure on Delta P-f is small. Correlations are also provided for the measured heat transfer coefficients and pressure drops in terms of the Nusselt number and friction factor, (C) 1998 Elsevier Science Ltd. All rights reserved.
Condensation heat transfer and pressure drop of refrigerant R-134a in a small pipe
(1999-02-01)
Yan, YY
;
Lin, TF
;
機械工程學系
;
Department of Mechanical Engineering
The characteristics of condensation heat transfer and pressure drop for refrigerant R-134a flowing in a horizontal small circular pipe that has an inside diameter of 2.0 mm were investigated experimentally in this study. The effects of the heat flux, mass flux, vapor quality and saturation temperature of R-134a on the measured condensation heat transfer and pressure drop were examined in detail. When compared with the data for a large pipe (D-i = 8.00 mm) reported in the literature, the condensation heat transfer coefficient averaging over the entire quality range tested here for the small pipe is about 10% higher. Moreover, we noted that in the small pipe the condensation heat transfer coefficient is higher at a lower heal flux, at a lower saturation temperature and at a higher mass flux. In addition, the measured pressure drop is higher for raising the mass flux but lower for raising the heat flux. Based on the present data, empirical correlation equations were proposed for the condensation heat transfer coefficient and friction factors. The results are useful in designing more compact and effective condensers for various refrigeration and air conditioning systems using refrigerant R-134a. (C) 1998 Elsevier Science Ltd. All rights reserved.
Condensation heat transfer and pressure drop of refrigerant R-410A flow in a vertical plate heat exchanger
(2005-12-01)
Kuo, WS
;
Lie, YM
;
Hsieh, YY
;
Lin, TF
;
機械工程學系
Heat transfer and associated frictional pressure drop in the condensing flow of the ozone friendly refrigerant R-410A in a vertical plate heat exchanger (PHE) are investigated experimentally in the present study. In the experiment two vertical counter flow channels are formed in the exchanger by three plates of commercial geometry with a corrugated sinusoidal shape of a chevron angle of 60 degrees. Downflow of the condensing refrigerant R-410A in one channel releases heat to the upflow of cold water in the other channel. The effects of the refrigerant mass flux, imposed heat flux, system pressure (saturated temperature) and mean vapor quality of R-410A on the measured data are explored in detail. The results indicate that the R-410A condensation heat transfer coefficient and associated frictional pressure drop in the PHE increase almost linearly with the mean vapor quality, but the system pressure only exhibits rather slight effects. Furthermore, increases in the refrigerant mass flux and imposed heat flux result in better condensation heat transfer accompanying with a larger frictional pressure drop. Besides, the imposed heat flux exhibits stronger effects on the heat transfer coefficient and pressure drop than the refrigerant mass flux especially at low refrigerant vapor quality. The friction factor is found to be strongly influenced by the refrigerant mass flux and vapor quality, but is almost independent of the imposed heat flux and saturated pressure. Finally, an empirical correlation for the R-410A condensation heat transfer coefficient in the PHE is proposed. In addition, results for the friction factor are correlated against the Boiling number and equivalent Reynolds number of the two-phase condensing flow. (c) 2005 Elsevier Ltd. All rights reserved.
Delayed onset of return flow by substrate inclination in model horizontal longitudinal MOCVD processes
(2005-01-15)
Kuo, WS
;
Wang, CY
;
Tuh, JL
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Lin, TF
;
機械工程學系
In this study an experimental flow visualization is carried out to investigate how the substrate inclination affects the buoyancy-induced return flow structure in mixed convection of gas in a horizontal rectangular duct. The return flow is driven by a heated circular disk embedded in the bottom plate of the duct. simulating that in a horizontal longitudinal MOCVD reactor. Specifically, the bottom plate of the duct is inclined so that the gas flow in the duct is accelerated, causing the buoyancy-to-inertia ratio to decrease in the main flow direction. In the experiment. the Reynolds and Rayleigh numbers of the flow at the duct inlet are respectively varied from 3.7 to 79.7 and from 90,40 to 24,000 for the inclined angle of the bottom plate fixed at 0degrees, 0.34degrees and 0.97degrees. Particular attention is paid to delineating the spatial changes of the return flow structure with the plate inclination angle and to how the bottom plate tilting possibly, suppresses and stabilizes the flow. The results show a substantial delay in the onset of the return flow and the effctive suppression of the buoyancy-driven unstable vortex flow by the bottom plate inclination. Besides, the bottom, plate inclination can effectively weaken the return flow at slightly higher Reynolds numbers. An empirical equation is provided to correlate the present data for the onset of the return flow in the duct with its bottom inclined at 0: and 0.97degrees. (C) 2004 Elsevier B.V. All rights reserved.
Effects of aspect ratio on vortex flow patterns in mixed convection of air through a bottom-heated horizontal rectangular duct
(1997-05-23)
Yu, CH
;
Lin, TF
;
機械工程學系
;
Department of Mechanical Engineering
Buoyancy-induced vortex flow structures and the associated heat transfer were numerically investigated in a mixed convective airflow in a bottom-heated horizontal rectangular duct of different aspect ratios. The unsteady three-dimensional Navier-Stokes and energy equations were directly solved by a higher order upwind finite difference scheme. Results were presented in particular for Reynolds numbers ranging from 5 to 15, Rayleigh numbers up to 9000, and aspect ratios from 4 to 12. The predicted results clearly show significant differences in vortex structures induced in ducts with small and large aspect ratios. For an aspect ratio less than 6 the transverse vortex rolls are periodically generated in the duct entry and gradually transform into longitudinal rolls when moving downstream. The resulting vortex flow eventually evolves to a time periodic state with the upstream and downstream portions of the duct dominated by the transverse rolls and longitudinal rolls, respectively. For a large aspect ratio (A > 9) the transverse rolls prevail in the duct core, with two to three longitudinal rolls existing near each sidewall. The flow oscillation in the region dominated by the transverse rolls is much higher than that dominated by the longitudinal rolls. At high Ra the flow becomes chaotic in time, and the duct is filled with unstable irregular vortex rolls.
Effects of jet-to-disk separation distance on the characteristics of mixed convective vortex flow in an impinging air jet confined in a cylindrical chamber
(2005-01-01)
Hsieh, JC
;
Lin, TF
;
機械工程學系
;
Department of Mechanical Engineering
How the jet-to-disk separation distance H affects various aspects of the laminar mixed convective vortex flow resulting from a low speed air jet impinging onto a heated horizontal disk confined in a vertical adiabatic cylindrical chamber is explored herein by flow visualization combined with temperature measurement. In the present study the air flow rate is varied from 1.0 to 8.0 slpm (standard liter per minute) for two different injection pipes with the temperature difference between the heated disk and air injected into the chamber varied from 0 to 25.0degreesC at four jet-to-disk separation distances. The corresponding jet Reynolds and Rayleigh numbers, respectively, range from 0 to 1082 and from 0 to 63,420. The experimental results clearly reveal a new vortex roll. More specifically, in the upper range of the jet Reynolds number tested here a small tertiary inertia-driven circular vortex roll appears near the upper wall of the chamber. in addition to the previously known primary and secondary inertia-driven rolls. Besides. we also note that a reduction in the jet-to-disk separation distance causes a delayed onset of the inertia-driven rolls except for H greater than or equal to 20.0 mm and in the meantime causes a substantial decrease in the Rayleigh number since Ra alpha H-3, which in turn significantly reduces the size and strength of the buoyancy-driven roll. Moreover, the inertia-driven rolls are bigger for a larger H. At the largest H (=30.0 mm) tested here both the primary inertia-driven roll and buoyancy-driven roll are relatively large and they can contact with each other. Hence no space is available for the secondary inertia-driven roll to appear. Our data also suggest that the onset of the buoyancy roll takes place when the local buoyancy-to-inertia ratio at the edge of the disk Gr/Re-we(2) approximate to 33.0 within the experimental uncertainty. Finally, the temperature data indicate that the steady radial air temperature distributions are non-monotonic which mainly reflects the counter-rotating vortex flow structure of the primary inertia-driven and buoyancy-driven rolls. (C) 2004 Elsevier.Ltd. All rights reserved.
Effects of thermal boundary condition on buoyancy driven transitional air flow in a vertical cylinder heated from below
(2000-06-01)
Cheng, TC
;
Li, YH
;
Lin, TF
;
機械工程學系
;
Department of Mechanical Engineering
A three-dimensional unsteady numerical computation was carried out here to investigate the effects of the thermal boundary condition on the convection flow in a vertical, bottom heated cylinder containing air. The thermal condition at the sidewall of the cylinder is assumed to be thermally well insulated or perfectly conducting. Results were obtained for air in a cylinder of finite aspect ratio (Gamma = 2) for various thermal Rayleigh numbers. The predicted results indicate that the flow in the sidewall insulated cylinder is highly asymmetric even at steady state and contains multicellular vortices. The flow formation processes leading to the above structures are relatively complicated. In the early transient two axisymmetric circular vortex rolls, one on top of another, appear. Then the rods merge asymmetrically. In the late stage the flow deflection by the cylinder top and bottom results in a very complex flow. In the cylinder with a perfectly conducting sidewall the transition from a steady to a time dependent flow is subcritical. However, in the cylinder with an insulated sidewall the flow transition is supercritical.
Enhancement of pool boiling heat transfer in a horizontal water layer through surface roughness and screen coverage
(1996-11-01)
Tsay, JY
;
Yan, YY
;
Lin, TF
;
機械工程學系
;
Department of Mechanical Engineering
An experimental investigation was carried out to study the augmentation of heat transfer in saturated pool boiling of a liquid water layer on a heated horizontal stainless steel plate by roughing the surface and/or covering it with a single layer of stainless steel screen. The results were presented in terms of the boiling curves. Effects of various parameters - the surface roughness, liquid level and size of the stainless steel mesh on the boiling heat transfer were examined in detail. The measured data clearly indicated that a lowering of the liquid level from 60 to 5 mm in water depth causes heat transfer reduction. Roughing the surface was found to significantly enhance the heat transfer. Use a layer of metal screen to cover the heated surface was shown to substantially augment the heat transfer especially for a shallow water layer if the mesh size is comparable with the bubble departure diameter. Covering the rough surface with the metal mesh, however, reduced the heat transfer.
Enhancement of pool boiling heat transfer in a horizontal water layer through surface roughness and screen coverage
(1996-11-01)
Tsay, JY
;
Yan, YY
;
Lin, TF
;
機械工程學系
;
Department of Mechanical Engineering
An experimental investigation was carried out to study the augmentation of heat transfer in saturated pool boiling of a liquid water layer on a heated horizontal stainless steel plate by roughing the surface and/or covering it with a single layer of stainless steel screen. The results were presented in terms of the boiling curves. Effects of various parameters - the surface roughness, liquid level and size of the stainless steel mesh on the boiling heat transfer were examined in detail. The measured data clearly indicated that a lowering of the liquid level from 60 to 5 mm in water depth causes heat transfer reduction. Roughing the surface was found to significantly enhance the heat transfer. Use a layer of metal screen to cover the heated surface was shown to substantially augment the heat transfer especially for a shallow water layer if the mesh size is comparable with the bubble departure diameter. Covering the rough surface with the metal mesh, however, reduced the heat transfer.
Evaporation heat transfer and pressure drop of refrigerant 134A in a small pipe - Reply
(2003-03-01)
Yan, YY
;
Lin, TF
;
機械工程學系
;
Department of Mechanical Engineering
Evaporation heat transfer and pressure drop of refrigerant R-134a in a plate heat exchanger
(1999-02-01)
Yan, YY
;
Lin, TF
;
機械工程學系
;
Department of Mechanical Engineering
The evaporation heat transfer coefficient and pressure drop for refrigerant R-134a flowing in a plate heat exchanger were investigated experimentally in this study. Two vertical counterflow channels were formed in the exchanger by three plates of commercial geometry with a corrugated sine shape of a chevron angle of 60 deg. Upflow boiling of refrigerant R-134a in one channel receives heat from the hot down flow of water in the other channel. The effects of the mean vapor quality mass flux, heat flux, and pressure of R-134a on the el,evaporation heat transfer and pressure drop were explored. The quality change of R-134a between the inlet and outlet of the refrigerant channel ranges from 0.09 to 0.18. Even at a very low Reynolds number, the present flow visualization of evaporation in a plate heat exchanger with the transparent outer plate showed that the flow in the plate heat exchanger remains turbulent It is found that the evaporation heat transfer coefficient of R-134a in the plates is much higher than that in circular pipes and shows a very different variation with the vapor quality from that bl circular pipes, particularly in the convective evaporation dominated regime at high vapor quality. Relatively intense evaporation on the corrugated surface was seen from the flow visualization. Moreover, the present data showed that both the evaporation hear transfer coefficient and pressure drop increase with the vapor quality. At a higher mass flux the pressure drop is higher for the entire range of the vapor quality but the evaporation heat transfer is clearly better only at the high quality. Raising the imposed wall heat flux was found to slightly improve the heat transfer, while at a higher refrigerant pressure, both the heat transfer and pressure drop are slightly lower. Based an the present data, empirical correlations for the evaporation heat transfer coefficient and friction factor were proposed.
Evaporation heat transfer and pressure drop of refrigerant R-134a in a small pipe
(1998-12-01)
Yan, YY
;
Lin, TF
;
機械工程學系
;
Department of Mechanical Engineering
An experiment was carried out to investigate the characteristics of the evaporation heat transfer and pressure drop for refrigerant R-134a flowing in a horizontal small circular pipe having an inside diameter of 2.0 mm. The data are useful in designing more compact and effective evaporators for various refrigeration and air conditioning systems. The effects of the imposed wall heat flux, mass flux, vapor quality and saturation temperature of R-134a on the measured evaporation heat transfer and pressure drop were examined in detail. When compared with the data for larger pipes (D-i greater than or equal to 8.0 mm) reported in the literature, the evaporation heat transfer coefficient for the small pipe considered here is about 30-80% higher for most situations. Moreover, we noted that in the small pipe the evaporation heat transfer coefficient is higher at a higher imposed wall heat flux except in the-high vapor quality region, at a higher saturation temperature, and at a higher mass flux when the imposed heat flux is low. In addition, the measured pressure drop is higher for increases in the mass flux and imposed wall heat flux. Based on the present data, empirical correlations were proposed for the evaporation heat transfer coefficients and friction factors. (C) 1998 Elsevier Science Ltd. All rights reserved.
Evaporation heat transfer and pressure drop of refrigerant R-134a in a small pipe
(1998-12-01)
Yan, YY
;
Lin, TF
;
機械工程學系
;
Department of Mechanical Engineering
An experiment was carried out to investigate the characteristics of the evaporation heat transfer and pressure drop for refrigerant R-134a flowing in a horizontal small circular pipe having an inside diameter of 2.0 mm. The data are useful in designing more compact and effective evaporators for various refrigeration and air conditioning systems. The effects of the imposed wall heat flux, mass flux, vapor quality and saturation temperature of R-134a on the measured evaporation heat transfer and pressure drop were examined in detail. When compared with the data for larger pipes (D-i greater than or equal to 8.0 mm) reported in the literature, the evaporation heat transfer coefficient for the small pipe considered here is about 30-80% higher for most situations. Moreover, we noted that in the small pipe the evaporation heat transfer coefficient is higher at a higher imposed wall heat flux except in the-high vapor quality region, at a higher saturation temperature, and at a higher mass flux when the imposed heat flux is low. In addition, the measured pressure drop is higher for increases in the mass flux and imposed wall heat flux. Based on the present data, empirical correlations were proposed for the evaporation heat transfer coefficients and friction factors. (C) 1998 Elsevier Science Ltd. All rights reserved.
Evaporation heat transfer and pressure drop of refrigerant R-410A flow in a vertical plate heat exchanger
(2003-10-01)
Hsieh, YY
;
Lin, TF
;
機械工程學系
;
Department of Mechanical Engineering
Experiments are carried out here to measure the evaporation heat transfer coefficient h(r) and associated frictional pressure drop DeltaP(f) in a vertical plate heat exchanger for refrigerant R-410A. The heat exchanger consists of two vertical counterflow channels which are formed by three plates whose surface corrugations have a sine shape and a chevron angle of 60 deg. Upflow boiling of refrigerant R-410A receives heat from the hot downflow of water. In the experiments, the mean vapor quality in the refrigerant channel is varied from 0.10 to 0.80, the mass flux from 50 to 100 kg/m(2)s, and the imposed heat flux from 10 to 20 kW/m(2) for the system pressure fixed at 1.08 and 1.25 MPa. The measured data indicate that both h(r) and DeltaP(r) increase with the refrigerant massflux except at low vapor quality. In addition, raising the imposed heat flux is found to significantly improve h(r) for the entire range of the mean vapor quality. However the corresponding friction factor f(tp) is insensitive to the imposed heat flux and refrigerant pressure. Based on the present data, empirical correlations are provided for h(r) and f(tp), for R-410A in the plate heat exchanger.