Heat Transfer Characteristics Of Refrigerant Research Articles

Numerical simulation on flow boiling heat transfer of R1234ze(E)/R152a in a horizontal smooth tube

The understanding of two-phase flow and heat transfer characteristics of refrigerants in tubes is important for guiding the design and optimization of heat exchangers. Based on the volume of fluid (VOF) multiphase model, this paper established a numerical model of R1234ze(E)/R152a (mass ratio of 0.4/0.6) flow boiling heat transfer in a horizontal smooth copper tube with an inner diameter of 6 mm and a length of 900 mm. The distribution of vapor volume fraction is obtained, and the influence of mass flux, heat flux, saturation temperature, and vapor quality on heat transfer coefficient (HTC) are studied. Bubble flow, plug flow, stratified flow, and wavy flow can be observed during the whole process and the fluid temperature increases along the tube. Local and time-averaged heat transfer coefficients and temperature distribution along the axial direction were studied. And the results indicate that the HTC decreases first and then increases with the augmentation of mass flux while increasing with the rise of heat flux. In addition, the HTC rises along with saturation temperature and decreases along with vapor quality. The largest related standard deviation between the simulation value and the testing data is 6.31%. Thus, the numerical simulation has a high level of accuracy.

Condensation heat transfer characteristics and generalized correlations of R404A, R448A, and R454C in a plate heat exchanger

Although numerous studies have been carried out on alternatives to R134a and R410A to cope with regulations, the heat transfer characteristics of refrigerants with a low-global warming potential to replace R404A in a plate heat exchanger (PHE) have not yet been intensively investigated. Generally, R448A has been considered as a short-term alternative to R404A, while R454C has been nominated as a long-term alternative to R404A. In this study, the thermal and hydrodynamic characteristics of R404A, R448A, and R454C in a PHE during condensation are experimentally investigated at vapor qualities (x) of 0.2–0.8, mass fluxes (G) of 15–35 kg m−2 s−1, heat fluxes (q”) of 3–7 kW m−2, and saturation temperatures (Tsat) of 20–27 °C. The results of the analysis show that the condensation heat transfer coefficient (HTC) and frictional pressure drop (FPD) are predominantly affected by the x and G, while the q” barely affects the FPD. The average condensation HTC and FPD of R448A and R454C exceed those of R404A by 17.25–20.57% and 9.6–10.72%, respectively. Additionally, new generalized correlations of the condensation Nusselt number and frictional pressure drop for R404A, R448A, and R454C in a PHE are developed with mean relative deviations of −1.905% and 1.01%, respectively.

Experimental determination of the role of roughness and wettability on pool-boiling heat transfer of refrigerant

In order to investigate pool boiling heat transfer characteristics of refrigerants on different surfaces, an experimental rig is established and verified in this study. The pool boiling heat transfer experiment is conducted with R134a at three saturation temperatures. Six surfaces with Ra roughness ranging from 0.133 to 5.742 μm and two types of wettability are tested in the study. The heat flux varies from 40 to 280 kW/m2. Experimental results indicate that higher roughness is favorable to boiling heat transfer. Shieldex coating on copper surfaces can deteriorate heat transfer by 20% approximately. Visualization analysis was carried out with a high-speed camera to study bubble behavior. A new-found mode of bubble departing was observed at lower heat flux. Besides, the data were correlated in the form of Rohsenow correlation, and comparison and supplement are concluded based on data obtained in this study.

Research on the offshore adaptability of new offshore ammonia-hydrogen coupling storage and transportation technology

Hydrogen energy and ammonia energy have received worldwide attention as ideal wind and solar energy storage media, which can effectively solve the problem of grid-connected deep-sea wind power and uneven spatial distribution of global energy. Accelerating the layout of the deep-sea ammonia energy industry is of great significance to build a global ammonia-hydrogen coupling interconnection system and ensure the security of energy supply. Ammonia storage and transportation is the key to offshore ammonia utilization. The new concept of LNH3 floating production storage and offloading unit (FLNH3) is proposed for the offshore ammonia energy gathering and transportation. Ammonia gas is directly liquefied at sea, which can simplify the storage and transportation process of ammonia energy. However, the sloshing condition of wind and waves at sea needs to be considered for the special environment of offshore operations. The offshore adaptability of the refrigerant in the floating ammonia liquefaction process system is particularly important. In this paper, a floating low-temperature heat transfer experimental device has been built to study the effect of complex sloshing conditions at sea on the flow and heat transfer characteristics of refrigerant in the liquid ammonia temperature zone. The results show that the sloshing motion has little effect on the heat transfer performance in the liquid ammonia temperature zone, and the process system shows strong marine adaptability.

Numerical study on the condensation characteristics of various refrigerants outside a horizontal plain tube at low temperatures

The condensation of refrigerants outside the tubes of liquefied natural gas (LNG) at low temperatures is the main factor affecting the performance of the LNG intermediate fluid vaporizer (IFV). In this paper, a CFD model is established to investigate the condensation heat transfer characteristics of refrigerants outside a horizontal plain tube at low temperatures over a wide range of wall subcooling. The model, based on the combination of the VOF multiphase model and Lee phase change model, agrees well with the experimental data. The transient condensation heat transfer coefficient is obtained and the periodic dripping characteristics of the condensate are analyzed. Due to the decrease of the interfacial mass transfer with the pressure decreasing, there is a peak of the periodic-averaged heat transfer coefficient with the variation of the saturation temperature, which is different from that of the Nusselt's prediction. The saturation pressures corresponding to the peak values of the periodic-averaged heat transfer coefficients at the subcooling of 10 °C are all in the range of 0.35–0.65 MPa for the four different refrigerants. In addition, the surface tension plays a positive role in the heat transfer process as the condensate film is pulled towards the wall and thinned. In view of the heat transfer performance along the whole LNG tube, dimethylether (DME) and butane can be considered as the intermediate fluids in the future design of the IFV by taking account of the comparison with six different refrigerants.

Evaporation heat transfer characteristics of R-245fa in a shell and plate heat exchanger for very-high-temperature heat pumps

A shell and plate heat exchanger (SPHE) has been introduced for use in very-high-temperature heat pumps (VHTHPs). However, studies on the evaporation heat transfer characteristics of refrigerants in SPHEs for use in VHTHPs are very limited. In this study, the evaporation heat transfer characteristics of R-245fa in the SPHE are measured and analyzed by varying the saturation temperature, mass flux, heat flux, mean vapor quality, and fluid direction. Experiments are conducted under high temperatures of 60−80 °C for VHTHP applications. The evaporation heat transfer coefficient increases as the mass flux and mean vapor quality increase, whereas it decreases as the heat flux increases. The two-phase frictional pressure drop increases as the mass flux and mean vapor quality increase. The heat transfer performance of the upward flow in the SPHE is better than that of the downward flow. In addition, empirical correlations of the Nusselt number and two-phase friction factor of R-245fa in the SPHE are derived.

Effect of SiO2 nanoparticles on the heat transfer characteristics of refrigerant and tribological behaviour of lubricant

Scientists and researchers are enthusiastically utilizing novel technologies that can substantially reduce energy consumption in diverse thermal systems. The foremost aim of the present investigation is to exploit the potential of nanoparticles on the enhancement of heat transfer characteristics of refrigerant and tribological capabilities of lubricant. Experimental investigations on the forced convection heat transfer characteristics of R134a/SiO2 nanorefrigerant (R134a + SiO2-polyalkylene glycol nanolubricant) are carried out. The effect of particle concentration, heat flux, mass flux, and vapour quality are investigated. The friction reduction and anti-wear properties of the nanolubricant are evaluated using four-ball tribo-tester. The results reveal that the presence of nanoparticles in the refrigerant significantly enhances the heat transfer coefficient of refrigerant and the tribological behaviour of lubricant. The maximum increase in heat transfer coefficient is 163.2% corresponding to a particle concentration of 0.4%. The presence of nanoparticles in refrigerant results in a marginal increase in pressure drop. However, the penalty in pressure drop is insignificant compared to the enhancement in heat transfer. Excellent friction reduction and anti-wear performance are also manifested by the nanolubricant. The maximum reduction in the friction coefficient is 37.76% and that of wear scar diameter is 40.83%. Considering the heat transfer and tribological characteristics, the optimum value of the particle volume fraction is 0.4%. SiO2-PAG nanolubricant can be considered as a potential energy-saving alternative to refrigerant compressor oils.

An overview of experimental studies on nanorefrigerants: Recent research, development and applications

Emergence of nanotechnology opens up new avenues in diverse research frontiers. Nanorefrigerants are nanotechnology based refrigerants, which are stable mixture of nanoparticles of metal, oxide, carbon and its allotropes and refrigerants. Development of nanorefrigerants provides a new research frontier in the heat transfer perspective. The studies on nanorefrigerants are still in its budding stage. The intention of the present paper is to provide a comprehensive overview of experimental studies on thermophysical and rehological properties, boiling and condensation phenomena, pressure drop characteristics, aggregation behaviour, migration and degradation characteristics of different nanorefrigerants and application of nanorefrigerants in HVAC systems. Various outlooks of future scope associated with the studies of nanorefrigerants have also been suggested. It has been recognized that addition of nanoparticles in refrigerants contributes noteworthy augment in the thermal physical and heat transfer characteristics of refrigerants. Furthermore, application of nanorefrigerants in heating, ventilation, and air conditioning systems enhance the overall performance in terms of coefficients of performance (COP) and energy efficiency.

Nanorefrigerants for energy efficient refrigeration systems

Nanorefrigerants, which are a new class of nanofluids, are mixtures of nanoparticles and refrigerants. They have several potential applications in various fields, such as for refrigeration and air-conditioning systems and heat pumps, among others. Accordingly, adding nanoparticles to conventional refrigerants results in anomalous improvements in the thermophysical properties and heat transfer characteristics of refrigerants, thereby further improving the performance of refrigeration systems. In the present study, R134a-based nanorefrigerant from R134a + CuO/Polyalkylene glycol (PAG) oil nanolubricant was prepared by dispersing copper oxide nanoparticles in the PAG lubricant. The effect of adding nanoparticles was investigated in relation to the thermal conductivity of lubricants and tribological characteristics of nanolubricants. The overall performance of a vapor compression refrigeration system with nanorefrigerants was evaluated. Enhanced heat transfer rate, coefficient of performance, freezing capacity, and reduced power consumption were observed for the refrigeration system. The thermal conductivity of the nanolubricant was better than that of the pure lubricant. Experimental results also showed that the low concentrations of CuO nanoparticles suspended in synthetic oil enhanced the tribological properties of the base oil.

Heat transfer measurements and correlation of refrigerant flow boiling in tube filled with copper foam

Flow boiling heat transfer characteristics of refrigerant in tubes filled with metal foams were experimentally investigated at a saturation temperature of 7 °C, mass flux from 10 to 90 kg m−2 s−1, vapor quality from 0.1 to 0.8 and heat flux from 3.1 to 18.3 kW m−2. The refrigerant is R410A. The test sections are tubes with inner diameter of 23.4 or 13.8 mm, filled with 5 or 10 PPI copper foam. The experimental results reveal that, metal foam enhances the flow boiling heat transfer by a maximum of 220% and promotes the flow pattern transition from stratified flow to stratified-wavy flow and from stratified-wavy flow to annular flow. The heat transfer coefficient decreases with vapor quality under low mass flux, while it increases with vapor quality under high mass flux. A correlation for predicting the flow boiling heat transfer was developed and it agrees well with the experimental data.

Influence of metal foam on heat transfer characteristics of refrigerant-oil mixture flow boiling inside circular tubes

The influence of metal foam on heat transfer characteristics of refrigerant and oil mixture flow boiling inside circular tubes was experimentally investigated on two horizontal circular tubes filled with 5 PPI and 10 PPI copper foams, respectively. The experimental conditions include saturation pressure of 995 kPa, mass flux from 10 to 30 kg m−2 s−1, heat flux from 3.1 to 9.3 kW m−2, inlet vapor quality from 0.175 to 0.775 and oil concentration from 0% to 5%. The experimental results reveal that metal foam enhances the flow boiling heat transfer coefficient of refrigerant by a maximum of 185%, while the enhancement effect of metal foam is weakened at oil presence conditions. A new correlation is developed, which can agree with 98% of the experimental data within a deviation of ±30%.

Impact of Channel Geometry on Two-Phase Flow Heat Transfer Characteristics of Refrigerants in Microchannel Heat Exchangers

Microchannel surfaces, often machined to 20 to 1000 μm in width and depth, are employed in high-heat-flux applications. However, a large number of variables, control the two-phase flow heat transfer coefficient. The pressure, the surface heat flux, and the mass flux significantly affect the thermal transport. Experiments were conducted on a setup that was built for testing microchannel heat exchanges. The parameters considered in the study are power input: 20 to 300 W, volume flow rate: 35 to 300 ml/min, quality: 0 to 0.5, inlet subcooling: 5 to 15°C. The results indicate that the heat transfer coefficient and pressure drop are functions of the flow quality, the mass flux, and, of course, the heat flux and the related surface superheat. The heat transfer coefficient decreases from a value of 12,000 W/m2-K to 9000, W/m2-K at 80°C, when the wall superheat is increased from 10 to 80°C. The coefficient decreases by 30 percent when the exit vapor quality is increased from 0.01 to 0.65.