T.F. Lin

Condensation heat transfer and pressure drop of refrigerant R-134a in a plate heat exchanger

Abstract An experimental refrigerant loop has been established in the present study to measure the condensation heat transfer coefficient h r and frictional pressure drop Δ 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°. 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 h r values for most cases. The corresponding rise in the Δ 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 Δ 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 Δ 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.

Saturated flow boiling heat transfer and pressure drop of refrigerant R-410A in a vertical plate heat exchanger

Abstract Saturated flow boiling heat transfer and the associated frictional pressure drop of the ozone friendly refrigerant R-410A (a mixture of 50 wt% R-32 and 50 wt% R-125) flowing in a vertical plate heat exchanger (PHE) are investigated experimentally in the 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°. Upflow boiling of saturated refrigerant R-410A in one channel receives heat from the downflow of hot water in the other channel. The experimental parameters in this study include the refrigerant R-410A mass flux ranging from 50 to 125 kg/m2 s and imposed heat flux from 5 to 35 kW/m2 for the system pressure fixed at 1.08, 1.25 and 1.44 MPa, which respectively correspond to the saturated temperatures of 10, 15 and 20 °C. The measured data showed that both the boiling heat transfer coefficient and frictional pressure drop increase almost linearly with the imposed heat flux. Furthermore, the refrigerant mass flux exhibits significant effect on the saturated flow boiling heat transfer coefficient only at higher imposed heat flux. For a rise of the refrigerant pressure from 1.08 to 1.44 MPa, the frictional pressure drops are found to be lower to a noticeable degree. However, the refrigerant pressure has very slight influences on the saturated flow boiling heat transfer coefficient. Finally, empirical correlations are proposed to correlate the present data for the saturated boiling heat transfer coefficients and friction factor in terms of the Boiling number and equivalent Reynolds number.

Evaporative cooling of liquid film through interfacial heat and mass transfer in a vertical channel. I, Experimental study

Abstract A numerical analysis is carried out to investigate the evaporative cooling of liquid falling film through interfacial heat and mass transfer in natural convection channel flows. Results for heat and mass transfer rates are specifically presented for the systems with ethanol film evaporation. The predicted results are also contrasted with the experimental results. The predictions show that the effects of the evaporative latent heat transfer on the cooling of the liquid film depend largely on the inlet liquid film temperature and liquid mass flow rate. Additionally, the results indicate that the heat transfer from the interface to the gas stream is predominated by the transport of the latent heat in conjunction with the liquid film evaporation.

Analysis of combined buoyancy effects of thermal and mass diffusion on laminar forced convection heat transfer in a vertical tube

This study investigates the role of vaporization of a thin liquid film onthe tube wall in laminar mixed convection flows under the combined buoyancy effects of thermal and mass diffusion. Results are specifically presented for an air-water system under various conditions. The effects of the liquid film temperature, the Reynolds number, and the relative humidity of the moist air in the ambient on the momentum, heat, and mass transfer in the flow are examined in great detail.

Combined heat and mass transfer in natural convection between vertical parallel plates with film evaporation

Abstract A numerical analysis is carried out to investigate the effects of latent heat transfer, in association with the evaporation of a finite liquid film on the channel wall, on the natural convection heat and mass transfer. Results for heat and mass transfer rates are specifically presented for the systems with ethanol film evaporation and water film evaporation. The predicted results with the transport in the liquid film treated are contrasted with those with the transport in the liquid film untreated and show that the assumption of an extremely thin film is only valid for a system with small liquid mass flow rate. But for a system with a large liquid mass flow rate, the assumption becomes inappropriate.

Cooling of a falling liquid film through interfacial heat and mass transfer

Abstract A numerical analysis was carried out to study the detailed heat transfer characteristics for a falling liquid ethanol film by solving the respective governing equations for the liquid film and the induced gas flow together. Meanwhile an experimental system was set up to measure the overall cooling of the film. The measured data are in good agreement with the numerical predictions. It was observed that the cooling of the liquid film is mainly caused by the latent heat transfer connected with the vaporization of the liquid film. Significant liquid cooling results for the system with a high inlet liquid temperature or a low liquid flowrate.

Structures of moving transverse and mixed rolls in mixed convection of air in a horizontal plane channel

A combined experimental and numerical investigation was carried out to explore the structures of the moving transverse and mixed vortex rolls in a low Reynolds number air flow through a bottom heated horizontal plane channel for Re 7.5 the transverse rolls occupied the core region and the longitudinal rolls exist near the side walls. For Re < 5.0 the entire duct is filled with the moving transverse rolls before they become distorted at very high Ra. A correlation equation for the wave speed of the transverse rolls was also proposed. Additionally, a flow regime map delineating different vortex flow patterns was given. The results from the numerical prediction qualitatively supported the experimental observation. Copyright 0 1996 Elsevier Science Ltd.

Transient conjugated heat transfer in fully developed laminar pipe flows

Abstract This study aims to numerically investigate the effects of conduction heat transfer in the pipe wall on the unsteady forced convective heat transfer in the flow in a long circular pipe resulting from a step change in uniform wall heat flux over a finite length of the pipe. Substantial influences are observed for the variations of Peclet number, radius ratio, conductivity ratio, and diffusivity ratio on the transient heat transfer characteristics. In particular, the wall-to-fluid heat capacity ratio is found to have a decisive impact on the unsteady heat transfer in the flow.

Experimental investigation of geyser boiling in an annular two-phase closed thermosyphon

Abstract Geyser boiling in a vertical annular two-phase closed thermosyphon was experimentally studied. The effects of the heat load, condenser temperature, degree of liquid fill and length of the evaporator on the characteristics of the geyser boiling were investigated in detail for both water and ethanol as working fluids. Flow visualization at light heat load clearly illustrates the process of geyser boiling. It also indicates that it occurs more frequently and irregularly at high heat load. The wall temperature measurement dictates that the period of geyser boiling is shorter at a higher heat load, a shorter evaporator length and a smaller liquid fill. Additionally, the heat transfer coefficient is found to increase approximately linearly with the heat load on a logarithmic scale. An empirical equation was proposed to correlate the data for the heat transfer coefficient. A criterion was suggested for the occurrence of geyser boiling based on the present data.

Subcooled flow boiling heat transfer of R-134a and bubble characteristics in a horizontal annular duct

Abstract Experiments were carried out to investigate the subcooled flow boiling heat transfer and visualize the associated bubble characteristics for refrigerant R-134a flowing in a horizontal annular duct having inside diameter of 6.35 mm and outside diameter of 16.66 mm. The effects of the imposed wall heat flux, mass flux, liquid subcooling and saturation temperature of R-134a on the resulting nucleate boiling heat transfer and bubble characteristics were examined in detail. In the experiment significant hysteresis was noted in the boiling curves during the onset of nucleate boiling (ONB) especially at low saturation temperature and high subcooling. The temperature undershoot at ONB is rather large for most cases. The boiling heat transfer was slightly higher for a lower saturation temperature and was little affected by the mass flux. However, for a higher subcooling of the refrigerant better heat transfer results. Furthermore, the flow visualization revealed that at higher imposed wall heat flux the heated surface was covered with more bubbles and the bubble generation frequency is higher. But the size of the bubbles departing from the heated surface was only slightly affected by the imposed heat flux. At high mass flux and subcooling the bubble generation was suppressed to a noticeable degree. Besides, the bubbles are much smaller at a higher subcooling. Finally, empirical correlations for the heat transfer coefficient and bubble departure diameter in the subcooled flow boiling of R-134a were proposed.