Evaporation Heat Transfer Of R-134a Research Articles

Experimental investigation on the heat transfer performance of falling film evaporation with lubrication oil on horizontal tubes having different structures

Falling film evaporation which has the advantages of refrigerant charge reduction and heat transfer improvement has been widely used in refrigeration and heat pump systems. Considering the inescapable and complicated influences of lubricating oil, the falling film evaporation heat transfer of R134a with POE-type lubricating oil entrained from 0.5% to 5.1% on horizontal tubes was examined. Experiments were conducted using test tubes with different surface structures at spray density of 0.0065–0.0105 kg/(s·m) and heat fluxes of 12–65 kW/m2. The saturation temperature was set to 6 °C. Experimental results showed that the heat transfer coefficients of the enhanced tubes were higher than that for the smooth tube and the lubricating oil could be conducive to the heat transfer of the tested tubes. The heat transfer coefficient of enhanced tube #1 was increased by more than 30% when the oil content was 5.1%. The mechanism of the function of lubricating oil on the heat transfer is closely related to the surface structure. For tube #1, oil content always supported heat transfer. In the case of tube #2, an optimal oil content at low spray density may exist.

Evaporation Heat Transfer of R134a on outside of Smooth and Enhanced Tubes

Increasing attention has been paid to the flow and heat transfer in annular channels since their intensive applications in engineering. In this study, evaporation of the outer surface of two enhanced heat transfer tubes (2EHT) was compared to the performance of a smooth surface copper tube through experimental research. The refrigerant flowed in an annular space between the outer tube and the test tube. The outside and inside diameters of tubes used in this study were 19.05 and 17.10 mm, respectively. Tests were performed using refrigerant R134a as the working fluid, over the saturation temperature of 6 °C for evaporation, the quality range of 0.1–0.8 and the mass fluxes is from 30 to 58 kg/m2s. It is observed that through different surface structures, the heat transfer area increased, while both the nucleate boiling and convective boiling were improved, leading to the heat transfer coefficient of 2EHT being significantly enhanced. According to the evaluation, the evaporation enhanced tube had better enhancement effect. Finally, correlation analysis was conducted based on experimental data. The results from correlation analysis can be implemented to predict the experimental heat transfer coefficients within a deviation of ±10%.

Heat transfer correlations of refrigerant falling film evaporation on a single horizontal smooth tube

Falling film evaporation heat transfer of R134a and its potential substitutes R290 and R600a outside a single horizontal plain tube is experimentally investigated, and the effects of the saturation temperature, film flow rate and heat flux on heat transfer coefficient are studied. Heat transfer performance of R290 is slightly superior than that of R134a, while R600a is inferior than that of R134a. The threshold film Reynolds number is determined to separate the variation trend of HTC with film Reynolds number into full wetting and partial dryout regimes. Increase of heat flux benefits the heat transfer in both full-wetting and partial dry-out regimes. New heat transfer correlations based on the present data and data for R32 and R1234ze(E) in the authors’ group are suggested for two regimes. The correlation for full wetting regime fits 96.7% of the total 542 correlated data within ±30% while fits 73.4% of the total 289 data in references from −30% to +15%, the correlation for partial dryout regime fits 97.5% of the total 162 correlated data within ±30% while fits 76.8% of the total 95 data from references within ±30%.

Two-Phase Flow Patterns and Evaporation Heat Transfer of R134a in a Vertical Narrow Rectangular Channel

This study experimentally investigated the flow pattern, void fraction, and evaporation heat transfer characteristic of R134a upward flow in a vertical narrow rectangular channel having a hydraulic diameter of 0.99[Formula: see text]mm, resembling a plate heat exchanger. Experiments were performed with mass velocities and vapor quality ranging between 30–200[Formula: see text]kgm[Formula: see text]s[Formula: see text] and 0.05–0.9, respectively, at a saturation temperature of 15[Formula: see text]C. Flow patterns were classified into bubble, slug, churn, and annular flows. The void fraction increased with increasing quality, and decreased with decreasing mass velocity in the low-quality region. Further, the influence of flow inlet/outlet positions remarkably appeared when the superficial gas velocity was low. The observed flow patterns and the measured void fraction were compared to that in previous studies. The effects of mass velocity and heat flux on the evaporation heat transfer coefficient were small, and the heat transfer through the thin liquid film was dominant.

Time periodic evaporation heat transfer of R-134a in a narrow annular duct due to mass flow rate oscillation

An investigation into the effect of mass flow rate oscillation on the R-134a evaporation heat transportation in a horizontal annular duct was experimentally conducted. The experiments were performed at different amplitudes (10, 20, and 30%) and periods (20, 30, 60, and 120 s) of the mass flux oscillation in a duct with different gap sizes (1.0, 2.0 and 5.0 mm). In this regard, the time variations of evaporation heat transportation coefficient and the heated wall temperature were also analyzed for the thermal characteristics of the oscillatory evaporation heat transfer. Measured results showed that the amplitude and period of the mass flux oscillation insignificantly affected the time-average heat transportation coefficient for the R-134a oscillatory evaporation heat transfer. However, the larger amplitudes and longer periods of the mass flux oscillation led to stronger wall temperature oscillations.

Experimental study on time periodic evaporation heat transfer of R-134a in annular ducts due to wall heat flux oscillation

An experimental study was carried out to examine the time periodic evaporation heat transfer for refrigerant R-134a flowing in horizontal narrow annular ducts subjected to wall heat flux oscillation in the form of triangular waves. The experiment was performed for the gap of the annular duct δ=1.0, 2.0 and 5.0mm. Besides, the amplitude of the imposed heat flux oscillation Δq was set at 0, 10, 30, 50, 80 and 100% of mean imposed heat flux q¯. Moreover, the period of the heat flux oscillation tp was fixed at 2, 20, 30, 60, 120 and 600s. Attention was mainly paid to examining the effects of the amplitude and period of the wall heat flux oscillation on the time periodic evaporation heat transfer in the narrow annular pipe. The measured evaporation heat transfer data are expressed in terms of the evaporation heat transfer coefficient. The thermal characteristics of the oscillatory evaporation heat transfer were illustrated by showing the time variations of the instantaneous heated pipe wall temperature Tw and evaporation heat transfer coefficient hr. Measured results showed that the time-average heat transfer coefficients for the oscillatory evaporation heat transfer of R-134a were not affected to a noticeable degree by the amplitude and period of the imposed heat flux oscillation. Besides, the stronger wall temperature oscillation was noticed for a longer period and a larger amplitude of the imposed heat flux oscillation and for a higher mean imposed heat flux.

Experimental study of evaporation heat transfer of R-134a inside a corrugated tube with different tube inclinations

Experimental heat transfer studies during evaporation of R-134a inside a corrugated tube have been carried out. The corrugated tube has been provided with different tube inclination angles of the direction of fluid flow from horizontal, α. The experiments were performed for seven different tube inclinations, α, in a range of −90° to +90° and four mass velocities of 46, 81, 110 and 136kgm−2s−1 for each tube inclination angle during evaporation of R-134a. Data analysis demonstrate that the tube inclination angle, α, affects the boiling heat transfer coefficient in a significant manner. The effect of tube inclination angle, α, on heat transfer coefficient, h, is more prominent at low vapor quality and mass velocity. In the low vapor quality region, the heat transfer coefficient, h, for the +90° inclined tube is about 62% more than that of the −90° inclined tube. The results also showed that at all mass velocities, the highest average heat transfer coefficient were achieved for α=+90°. An empirical correlation has also been developed to predict the heat transfer coefficient during flow boiling inside a corrugated tube with different tube inclinations.

Evaporation heat transfer of R-134a inside a microfin tube with different tube inclinations

An experimental investigation has been carried out to study the heat transfer characteristics during evaporation of R-134a inside a single helical microfin tube. The microfin tube has been provided with different tube inclination angles of the direction of fluid flow from horizontal, α. The experiments were performed for seven different tube inclinations, α, in a range of −90° to +90° and four mass velocities of 53, 80, 107 and 136 kg/m 2 s for each tube inclination angle during evaporation of R-134a. The results demonstrate that the tube inclination angle, α, affects the boiling heat-transfer coefficient in a significant manner. For all refrigerant mass velocities, the best performing tube is that having inclination angle of α = +90°. The effect of tube inclination angle, α, on heat-transfer coefficient, h, is more prominent at low vapor quality and mass velocity. An empirical correlation has also been developed to predict the heat-transfer coefficient during flow boiling inside a microfin tube with different tube inclinations.