Boiling Heat Transfer Of R134a Research Articles

Flow Boiling Heat Transfer of R134a at Varying Heat Fluxes

Flow Boiling Heat Transfer of R134a at Varying Heat Fluxes

Flow Boiling Heat Transfer of R134a in a Horizontal Smooth Tube: Experimental Results, Flow Patterns, and Assessment of Correlations

This study presents an extensive evaluation of heat transfer characteristics, flow patterns, and pressure drop for saturation pressures ranging from 460–660 kPa in a horizontal smooth tube of 5 mm internal diameter using R134a as the working fluid. The effect of saturation pressures for mass fluxes of 150–300 kg/m2s and heat fluxes of 8.26–23.3 kW/m2 which are typical of refrigeration and air conditioning applications are also investigated. Flow patterns observed during the study are predicted with a well-known flow pattern map of Wojtan et al. The experimental results are compared with seven (7) correlations developed based on different theories to find which correlation best predicts the experimental data. The results show that, at low mass flux, increasing saturation pressure results in an increased heat transfer coefficient. This effect is more pronounced in the low vapor quality region and the dominant mechanism is nucleate boiling. At high mass flux, increasing saturation pressure leads to an insignificant increase in the heat transfer coefficient. At this high mass flux but low heat flux, the heat transfer coefficient increases with vapor quality, indicating convective boiling dominance. However, for high heat flux, the heat transfer coefficient is linear over vapor quality, indicating nucleate boiling dominance. Pressure drop is observed to decrease with increasing saturation pressure. Increasing saturation pressure increases the vapor quality at which the flow pattern transitions from intermittent flow to annular flow. The flow patterns predicted are a mixture of slug and stratified wavy and purely stratified wavy for low mass fluxes. For increased mass fluxes, the flow patterns predicted are slug, intermittent, annular, and dryout. Cooper’s model was the best predictor of the experimental data and the trend of heat transfer coefficient.

Flow Boiling Heat Transfer of R134a in a Vertical Helically Coiled Tube

ABSTRACTThe boiling heat transfer characteristics of R134a in a helically coiled tube under high heat flux condition were investigated experimentally in present paper. The inner diameter and the coil diameter of the coil were 8 mm and 205 mm, respectively. Experiments were carried out with the heat flux in the range of 10–60 kW/m2, pressure 0.8–1.1 MPa, mass flux 195–400 kg/(m2 s) and vapor qualities 0.1–0.9. The results show that nucleate boiling plays a significant role even at high vapor quality. Inversely, the convective boiling was weakened in the high quality region under high heat flux conditions. The available boiling heat transfer coefficient correlations were compared with present experimental results. A new boiling heat transfer coefficient correlation for R134a in helically coiled tube was proposed based on the boiling mechanisms analysis. The new correlation was also proved to be applicable for high mass flux and low heat flux conditions.

Experimental Study of Horizontal Flow Boiling Heat Transfer of R134a at a Saturation Temperature of 18.6 °C

In spite of the extensive work in flow boiling in small-diameter tubes, the general characteristics and dominant mechanisms remain elusive. In this study, flow boiling heat transfer of R134a inside a 5 mm I.D., smooth horizontal stainless steel pipe is experimentally studied. Local heat transfer coefficients (HTCs) were measured for heat fluxes from 3.9 to 47 kW/m2 and mass fluxes from 200 to 400 kg/m2 s at a saturation temperature of 18.6 °C. The studied cases have shown different behaviors at low and high heat fluxes. At low heat fluxes, the convective contribution looks to control the HTC, while at high heat fluxes the nucleation of vapor looks to be the dominant mechanism. Reducing the heat flux, the HTC approaches asymptotically a limit equivalent to the single-phase HTC defined in terms of the sum of the superficial liquid and vapor Reynolds numbers. A new correlation for dominant convective flow boiling is proposed and evaluated against experimental data from the literature.

Experimental Investigation of Nucleate Pool Boiling Heat Transfer of R134a on TiO2 Coated TF Surface

Now-a-days, to save energy and to save environment are the great issues of the world. The enhancement of boiling heat transfer is one of the most important and advanced research fields to meet up the partial crisis of energy. But the way of research should be environmentally friendly. Various inferences have been drawn based on the existing parameters like operating pressure, temperature, boiling surface roughness, surface orientation, and types of working fluids or nano fluids by different researchers for enhancement of heat transfer rate. Here is an experimental study which is conducted to investigate the pool boiling heat transfer of environmentally friendly refrigerant R134a liquid at pool temperature 25 on flat copper surfaces. The surfaces are fabricated with titanium dioxide (TiO2) thin film (TF) coating of thickness 100 nm and 200 nm. The Electron Beam Evaporator (EBE) is used to fabricate the thin film on test surfaces. The study is conducted on a set of values of heat fluxes of each experiment to clarify the effect of the heat flux on test surface. The surfaces are characterized with roughness, film thickness and scanning electron microscopy (SEM) & atomic force microscopy (AFM) etc. The results are compared with a plain copper surface and observed that the maximum enhancement heat transfer coefficient comes from 200 nm coating thickness of substrate and minimum from 100 nm. The heat transfer coefficient of coated surfaces was better than uncoated surface due to augmented roughness and increment of dynamic nucleation site density.

Empirical correlations for the determination of R134a's convective heat transfer coefficient in horizontal and vertical evaporators having smooth and corrugated tubes

Boiling heat transfer of R134a flowing in the inner tube and hot water flowing in the annulus were examined inside smooth and corrugated tube-in-tube heat exchangers in authors' previous studies experimentally. The apparatus has 0.85m long double tube for vertical orientation and 2.5m long double tube for horizontal orientation as its test sections whose inner ones are smooth and corrugated copper tubing having inner diameters of 0.0087m for vertical tubes and 0.0081m for horizontal tubes. The range of mass fluxes are 20,300 and 400kgm−2s−1 for vertical tubes, and 300, 400 and 500kgm−2s−1 for horizontal tubes. The average vapor qualities vary from 0.14 to 0.86 for vertical tubes, 0.09–0.81 for horizontal tubes, and saturation pressure interval is between 4.5 and 5.7 for both tube orientations. The average boiling heat transfer coefficient of R134a is calculated by means of the energy balance in the test section. In this existing study, the predictability of almost whole well-known empirical correlations in literature suggested for convective boiling flows in smooth and corrugated tubes is experienced by using the authors'large number of experimental database (302 data points for vertical tubes and 227 data points for horizontal tubes). Characteristics of trend lines plotted for the change of vapor quality, saturation temperature, mass flux and heat flux with experimental boiling heat transfer coefficients are examined as well in relation to various saturation temperatures and mass fluxes. Furthermore, the most successful correlations validated their predictabilities for both vertical and horizontal oriented evaporators having smooth and corrugated tubes.

Correlations of Flow Boiling Heat Transfer of R-134a in Minichannels: Comparative Study

R-134a is one of the most widely used refrigerants, and minichannel refrigeration systems with R-134a have rapidly developed in many fields, such as home, automobile and aircraft air conditioning systems, for high efficiency operations to save energy and space. A number of correlations for flow boiling hear transfer have been proposed. There is some literature to evaluate existing correlations for R-134a flow boiling heat transfer in minichannels. However, they were only based on the authors own experimental data. Therefore, results are often not consistent, even controversial. Our efforts are devoted to develop a better flow boiling heat transfer correlation for R-134a in minichannels, and this paper presents the first part of our efforts: A comparative study of existing correlations for flow boiling hear transfer of R-134a in minichannels. From 9 published papers, 1158 data points of flow boiling heat transfer of R-134a in minichannels are collected. Eighteen flow boiling heat transfer correlations, including almost all well-known ones, are reviewed and compared with the data collected. It is found that no correlation has satisfactory accuracy. The best one has a mean absolute relative deviation above 36%. It is interesting to note that among the six best correlations, one was developed for pool boiling and two were developed for conventional channels, and most of correlations developed specially for minichannels do not work quite well. More efforts should be made to better understand the mechanism of flow boiling heat transfer in minichannels for developing better correlations. Key words: R-134a; Flow boiling; Heat transfer; Correlation; Minichannel

Nucleate Pool Boiling Heat Transfer of R134a and R134a-PVE Lubricant Mixtures on Smooth and Five Enhanced Tubes

Pool boiling heat transfer coefficients of R134a with different lubricant mass fractions for one smooth tube and five enhanced tubes were tested at a saturation temperature of 6°C. The lubricant used was polyvinyl ether. The lubrication mass fractions were 0.25%, 0.5%, 1.0%, 2.0%, 3.0%, 5.0%, 7.0%, and 10.0%, respectively. Within the tested heat flux range, from 9000 W/m2 to 90,000 W/m2, the lubricant generally has a different influence on pool boiling heat transfer of these six tubes.

The Effect of Novel Plasma-Coated Compact Tube Bundles on Pool Boiling

Boiling heat transfer of R-134a on a porous, plasma-coated tube bundle was investigated experimentally to determine the effects of the number of tube rows and the total tube number. The bundle consists of up to four tubes with a pitch-to-diameter-ratio of 1.33. Heat transfer coefficients for a single tube with a porous copper coating were up to four times higher than for a smooth tube. Observations showed that the plasma coating enhanced the heat transfer coefficient in tube bundles as well. The bundle factor of the coated tube bundle showed a similar, slightly increased trend as the smooth tube bundles. The enhancement effect of the coated tubes decreases to a certain extent with an increasing heat flux and decreasing saturation temperature. However, it is significantly less pronounced than trends that have been reported from other investigations. The aim of a stable enhanced coating was confirmed by long-term experiments with steady results.