R134a Vapor Research Articles

Transient characteristics of flow boiling in large micro-channel heat exchangers

This study investigates both time-averaged and transient characteristics of large area micro-channel evaporators incorporated into an R-134a vapor compression loop that simulates thermal control for future space vehicles. The loop contains two separate micro-channel evaporators, one to simulate heat acquisition from the spacecraft crew and the other from the avionics. Both evaporators feature parallel 1×1-mm2 micro-channels, with the smaller crew evaporator yielding relatively low qualities, and the larger avionics evaporator both low and high qualities. Heat transfer measurement and high speed video are used to investigate variations of heat transfer coefficient with quality for different mass velocities and heat fluxes, as well as transient fluid flow and heat transfer behavior. Relatively low qualities in the crew evaporator are dominated by slug flow and the nucleate boiling mechanism. On the other hand, the avionics evaporator produces different flow regimes and heat transfer mechanisms depending on quality range, with low qualities associated with slug flow and dominated by nucleate boiling, and high qualities by annular flow and convective boiling. Further increases in quality trigger incipient dryout of the annular film and transition to mist flow. An important transient phenomenon that influences both fluid flow and heat transfer is a liquid wave composed of remnants of liquid slugs from the slug flow regime. The liquid wave serves to replenish dry wall patches in the slug flow regime and to a lesser extent the annular regime. Two-phase flow in the two evaporators shows clear signs of parallel-channel instability.

Influence of cycle time and collector area on solar driven adsorption chillers

Dynamic performance of a single-stage, two-bed, silica gel+water adsorption chiller operating in Bangalore, India is studied. Driving thermal energy is provided directly by an evacuated tube solar collector field. System dynamics are evaluated in the absence of thermal storage, which causes intra-day fluctuations in heat source and evaporator temperatures, which in turn influence the system performance. These dynamics are demonstrated for representative days in the months of April (summer) and December (winter). The focus is on the effect of variation of the collector area and the adsorption cycle time on the system performance. The maximum temperature of heat transfer fluid (water) is limited to 95°C. The cyclic and daily averages of solar coefficient of performance (DACOPsol) and cooling capacity (DACC) are used as key performance indicators. One of the key aspects of the this study is to show that both of them can be maximized by suitably choosing the collector area and cycle time. Further, it is demonstrated that the solar driven adsorption chiller described here is ideally suited for cascading with an air-cooled R-134a vapour compression refrigeration system (VCRS). The variable throughput obtained from the solar adsorption chiller can help in liquid sub-cooling and hence to cover the deficit in cooling capacity of the VCRS arising due to high ambient temperature.

Liquid–vapor phase distribution in horizontal headers with upward minichannel-branching conduits

Since the refrigeration system based on carbon dioxide (CO2) as a refrigerant is near ideal, liquid–vapor phase distribution in a header type evaporator applied for CO2 has been experimentally investigated. The test section consists of a horizontal header and 3, 6 or 8 vertically upward replaceable branching conduits. The header is made of transparent polyvinyl chloride (PVC) resin for observing the flow regime, and each conduit is composed of an aluminum disk plate and an aluminum minichannel-branching conduit. In order to develop the lowest maldistribution header, several headers with various insertion depths are examined. Since the CO2 system operates at around the supercritical conditions, it is a little bit difficult to observe the flow regime for CO2 at high pressure, therefore, R134a is used as the alternative working fluid in this study. Similarity hypothesis between CO2 and R134a is proposed. The modified Baker flow pattern map (refer to Scott, 1963) is applied to define the equivalent experimental conditions for both R134a and CO2. R134a headers are tested and the experimental work is applicable to CO2. The R134a experiments were conducted at saturation temperature of about 21°C, refrigerant mass flow range between 10 and 30kg/h which corresponds to about 44 and 130kg/m2s in the 9mm i.d. header, and average vapor quality in the test section inlet of about 0.1, 0.2, 0.3 and 0.4. The R134a vapor–liquid phase mass flows entering into the branches were measured. The predicted flow regimes at each branch inlet in the header, predicted using flow pattern map in the form of vapor quality as the abscissa versus mass velocity as the ordinate refer to Wojtan et al. (2005), were utilized to understand the observation of phase separation. The liquid–vapor phase distribution of CO2 was estimated based on the R134a experimental result.

Proposed split-type vapor compression refrigerator for heat hazard control in deep mines

The thermodynamic performance of a proposed split-type vapor compression refrigerator (SVCR) for heat hazard control in a deep mine was numerically analyzed. The thermodynamic characteristics of the suction process of the proposed system were also simulated and analyzed. In the suction direction, the pressure, velocity, temperature, and specific enthalpy of R134a vapor dropped, whereas its specific volume and specific entropy rose. The variations in the superheat of the compressor inlet and the characteristics of the pressure drop of the suction process were also described in detail and analyzed. For comparison with the theoretical cycle, a variable, γ, defined as the COP ratio of the proposed system and the theoretical cycle, was presented, and the variations of γ were studied and discussed in detail. The proposed system was compared with a cooling water system and an ice slurry system, and results indicate that the thermal performance of the proposed system is better than that of the cooling water system. The thermal performance of the ice slurry system is better than that of the proposed system when a mine is sufficiently deep. However, when the mine is not too deep and the evaporation temperature, TE, is not too low, the proposed SVCR system is the better choice.

Forced Convective Condensation of R134A Vapor Inside a Vertical Microfin Tube

In this study, condensation of pure refrigerant R134a vapor inside a vertical 18° helical microfin tube was experimentally investigated. Tests were performed at saturation pressure of 5.7–5.9 bar with mass fluxes of 20–100 kg/m2s and heat fluxes of 1.7–5.3 kW/m2. The effects of mass flux and the temperature difference between the refrigerant and tube wall (ΔT) on the heat transfer performance were analyzed throughout experimental data. For experiments in which ΔT is more than 2.5°C, the average condensation Nusselt number showed a tendency to be independent from ΔT. Heat transfer enhancement ratio was found to be 1.59–1.71, which is always higher than the heat transfer area enhancement factor (1.55). Fins always act as a turbulence promoter in the given experimental data range. Finally, the most widely used heat transfer coefficient correlations for condensation inside microfin tubes were analyzed through the experimental data. Best fit was obtained with Yu and Koyama's correlation with an absolute mean deviation of 17% and Kedzierski and Goncalves's correlation with an absolute mean deviation of 19%.

Heat Transfer in Rotating Serpentine Coolant Passage With Ribbed Walls at Low Mach Numbers

This paper experimentally investigates the effect of rotation on heat transfer in typical turbine blade serpentine coolant passage with ribbed walls at low Mach numbers. To achieve the low Mach number (around 0.01) condition, pressurized Freon R-134a vapor is utilized as the working fluid. The flow in the first passage is radial outward, after the 180 deg tip turn the flow is radial inward to the second passage, and after the 180 deg hub turn the flow is radial outward to the third passage. The effects of rotation on the heat transfer coefficients were investigated at rotation numbers up to 0.6 and Reynolds numbers from 30,000 to 70,000. Heat transfer coefficients were measured using the thermocouples-copper-plate-heater regional average method. Heat transfer results are obtained over a wide range of Reynolds numbers and rotation numbers. An increase in heat transfer rates due to rotation is observed in radially outward passes; a reduction in heat transfer rate is observed in the radially inward pass. Regional heat transfer coefficients are correlated with Reynolds numbers for nonrotation and with rotation numbers for rotating condition, respectively. The results can be useful for understanding real rotor blade coolant passage heat transfer under low Mach number, medium–high Reynolds number, and high rotation number conditions.

Performance evaluation of an R134a automotive heat pump system for various heat sources in comparison with baseline heating system

Engine waste heat is commonly used for comfort heating and defogging windows of the passenger compartment of motor vehicles. However, the amount of waste heat to be used for these purposes decreases continuously as a result of increasing engine efficiencies. These requirements can also be met by employing an automotive heat pump (AHP) system. This study presents performance characteristics of an experimental AHP system employing an R134a vapour compression refrigeration circuit and driven by a diesel engine. The AHP system is capable of providing a conditioned air stream by utilizing the heat absorbed from any of the three sources, namely ambient air, engine coolant and exhaust gas. The steady-state and transient performance of the AHP system for each heat source was evaluated by applying energy and exergy analysis to the system based on experimental data. Then, a performance comparison of the AHP system with the three heat sources and baseline heating system was made. It was determined that the AHP system using engine coolant provided the highest indoor coil outlet air temperature during the tests until the steady-state was achieved compared with the AHP system using other heat sources as well as the baseline heating system.

Experimental study on film condensation of superheated vapour on a horizontal tube

Starting from a proposed theoretical two-phase boundary model for the film condensation of ambient air on a cryogenic horizontal tube (Zhao et al., 2014), we conducted an experimental study on the film condensation of superheated vapours for further study. Condensation experiments were performed for the R134a vapour with various superheating outside a horizontal tube cooled by inside flowing cryogenic nitrogen gas. The reduction of experimental data to obtain condensation-side heat transfer coefficient was accomplished by using two different approaches, individually, which were tube wall measurement technique and modified Wilson plot technique. The results obtained under different conditions were compared with each other, in which a certain divergence existed acceptably. The result shows that the condensation heat transfer coefficient increases by 9.79% when the superheating changes from 39.5K to 131.9K. Meanwhile, the condensation heat transfer performance becomes worse due to the combination of condensation and natural convection as the cooling capacity inside the tube is smaller than the critical heat flux. This phenomenon is only available for the condensation of superheated vapour especially for the higher superheating. The condensation heat transfer coefficients from experimental measurement in this paper are compared with the simulated results from the two-phase boundary model previously presented, and a good agreement is reached.

Heat Transfer Characteristics for Condensation of R134a in a Vertical Smooth Tube

In this study, condensation of pure refrigerant R134a vapor inside a smooth vertical tube was experimentally investigated. The test section was made of a copper tube with inside diameter of 7.52 mm and length of 1 m. Experimental tests were conducted for mass fluxes in the range of 20–175 kg/m2s with saturation pressure ranging between 5.8 and 7 bar. The effects of mass flux, saturation pressure, and temperature difference between the refrigerant and tube inner wall (ΔT) on the heat transfer performance were analyzed through experimental data. Obtained results showed that average condensation heat transfer coefficient decreases with increasing saturation pressure or temperature difference (ΔT). In addition, for the same temperature difference (ΔT), heat can be removed from the refrigerant at a higher rate at relatively low pressure values. Under the same operating conditions, it was shown that average condensation heat transfer coefficient increases as mass flux increases. Finally, the most widely used heat transfer coefficient correlations for condensation inside smooth tubes were analyzed through the experimental data. The best fit was obtained with Akers et al.'s (1959) correlation with an absolute mean deviation of 22.6%.

A comparison of condensation heat transfer performance of MWCNT/Fe composite coatings on steel substrate

In the present study, the effect of MWCNT/Fe coatings on steel substrate was investigated using R134a vapor. Specifically, CNT and Fe2O3 powder were used as components in mechanically alloyed powder composites, which were used to fabricate surface coatings. There were two types of coatings used in the tests. The first one was pure Fe coating and the second one was the MWCNT/Fe composite coating. The powders were coated on a plain carbon steel plate by electrostatic spraying. This was followed by sintering at 900 °C. Results showed that the condensation heat transfer can be enhanced by MWCNT/Fe composite coatings, while a pure Fe coating can diminish the enhancement in comparison to the plain carbon steel plate.

Heat Transfer Measurements in Rotating Blade–Shape Serpentine Coolant Passage With Ribbed Walls at High Reynolds Numbers

Flow in the internal three-pass serpentine rib turbulated passages of an advanced high pressure rotor blade is simulated on a 1:1 scale in the laboratory. Tests to measure the effect of rotation on the Nusselt number are conducted at rotation numbers up to 0.4 and Reynolds numbers from 75,000 to 165,000. To achieve this similitude, pressurized Freon R134a vapor is utilized as the working fluid. Experimental heat transfer coefficient measurements are made using the copper-plate regional average method. Regional heat transfer coefficients are correlated with rotation numbers. An increase in heat transfer rates due to rotation is observed in radially outward passes; a reduction in heat transfer rate is observed in the radially inward pass. Strikingly, a significant deterioration in heat transfer is noticed in the “hub” region—between the radially inward second pass and the radially outward third pass. This heat transfer reduction is critical for turbine cooling designs.

Demonstration of a wearable cooling system for elevated ambient temperature duty personnel

A wearable cooling system was developed for use in elevated temperature environments by military, fire-fighting, chemical-response, and other hazardous duty personnel. The cooling system consists of an engine-driven R134a vapor compression system assembled in a backpack configuration, coupled with a cooling garment containing refrigerant lines. A 2.0 L fuel tank powers a small-scale engine that runs a compressor fabricated in house. The overall cooling system, including the wearable evaporator, had a mass of 5.31 kg and measured 0.318 × 0.273 × 0.152 m. Controlled environment tests determined system performance over a range of ambient temperatures (37.7–47.5 °C), evaporator refrigerant temperatures (22.2–26.1 °C), and engine speeds (10,500–13,300 RPM). Heat removal rates of up to 300 W, which is the cooling rate for maintaining comfort at an activity level comparable to calisthenics or moderate exercise, were demonstrated at an ambient temperature of 43.3 °C. The system consumed 1750 W at a fuel flow rate of 0.316 kg h−1 to provide a 178 W of cooling for 5.7 h.

Experimental analysis of R1234yf as a drop-in replacement for R134a in a vapor compression system

This paper presents an experimental analysis of a vapor compression system using R1234yf as a drop-in replacement for R134a. In this work, we compare the energy performance of both refrigerants, R134a and R1234yf, in a monitored vapor compression system under a wide range of working conditions. So, the experimental tests are carried out varying the condensing temperature, the evaporating temperature, the superheating degree, the compressor speed, and the internal heat exchanger use. Comparisons are made taking refrigerant R134a as baseline, and the results show that the cooling capacity obtained with R1234yf in a R134a vapor compression system is about 9% lower than that obtained with R134a in the studied range. Also, when using R1234yf, the system shows values of COP about 19% lower than those obtained using R134a, being the minor difference for higher condensing temperatures. Finally, using an internal heat exchanger these differences in the energy performance are significantly reduced.

Investigations on R134a–DMAC vapour absorption refrigeration system with add-on components

R134a (1, 1, 1, 2 tetrafluro ethane)–DMAC (N, N Dimethyl Acetamide) vapour absorption refrigeration system (VARS) can be used for sub–zero temperature applications and in industries where ammonia is forbidden. Because of the low boiling temperature difference between R134a and DMAC, a little amount of the latter also boils along with the former in the generator. Hence, this system needs a rectifier. Owing to incomplete rectification, a little amount of DMAC escapes to the condenser which ultimately results in residual liquid in the evaporator and cooling loss. This paper evaluates the performance of R134a–DMAC VARS with rectifier and liquid vapour heat exchanger (LVHX) at different operating conditions by simulation studies. The system performance becomes maximum at some optimum generator temperature which increases with sink temperature and decrease in cooling temperature. Owing to a large heat duty of solution heat exchanger (SHX), its efficiency is vital in increasing the system COP. Due to the presence of the residual liquid and low ratio of latent heat to specific heat of R134a vapour, LVHX plays an important role. Rectifier loses its importance when efficiency of LVHX is high. Significance of all the three components increases at low cooling and high sink temperatures.

Visual study of flow patterns during condensation inside a microfin tube with different tube inclinations

In this paper, flow patterns and their transitions for refrigerant R-134a condensing in a microfin tube are visually observed and analyzed. 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 refrigerant mass velocities in a range of 53 to 212 kg/m 2s for each tube inclination angle during condensation of R-134a vapor. From analysis of acquired data, it was found that the tube inclination strongly influenced the vapor and condensate liquid distribution. Annular flow was the dominant flow pattern for vertical downward flow, α = − 90°. Annular flow, semi annular flow and stratified flow were observed for α = − 60°and − 30°. Annular flow, wavy-annular flow and stratified-wavy flow exist in sequence for horizontal tube. Annular flow and wavy-annular flow were observed for α = + 30°and + 60°. Annular flow, annular-wavy flow, churn flow and slug flow occurred for α = + 90°.

Experimental performance of a compliant scroll expander for an organic Rankine cycle

A small-scale scroll expander was developed and tested. The expander was based on a compliant scroll compressor having a displacement of ∼6.8 cm3. A test loop was also developed for determining expander performance parameters. The loop employed a compressor to pressurize the working fluid up to 2758 kPa and a heat exchanger to create the expander inlet conditions that would characterize an operating power cycle. The expander was tested with R134a as the working fluid. For power output measurements, a dynamometer was used where torque and rotational rate were measured. An important feature in adapting a scroll compressor to expander duty was the lubrication of the scroll wraps during operation and the lubrication required by the journal bearings within the expander housing. The approach used in this study was to dissolve a compatible oil into the circulating working fluid to lubricate the wraps, while a pump was employed to deliver pressurized oil to the bearings. The pressurized oil supply was also used to generate a controlled force to the upper stationary scroll wrap through a sealed piston assembly. This force maintained axial contact between the upper and lower scroll wraps during operation. After expansion, the R134a vapour passed through an orifice flow meter and then to a liquid-cooled heat exchanger. It then enters a compressor and heat exchanger to complete the flow loop. A series of performance tests were conducted on the expander at various expander rotational speeds, inlet and outlet pressures, and scroll sealing pressures. Test results show consistent isentropic efficiencies of over 70 per cent for the expander at its maximum efficiency operating points. The expander performance, however, is a function of speed, pressure ratio, and scroll sealing pressure.

Numerical simulation of churn flow in a vertical pipe

A numerical simulation of the churn flow regime of air–water and R134a vapour–liquid mixtures by means of the volume of fluid (VOF) method is presented. The focus of the paper is on the inlet region of a vertical pipe. An axisymmetrical domain is used, reproducing the region next to the porous wall liquid injector of a typical test section for the investigation of vertical gas–liquid flows. A simplified model of the levitation process of the ring-type waves typical in churn flow is proposed. The influence of the gas Froude number on the waves amplitude is shown by means of the simplified model and used to explain the numerical results. A comparison of the numerical results with experimental wave frequency data and visualizations available in the literature is performed. The velocity field in the forming wave region and the pressure and shear stress variations along the interface are shown. Simulations have been performed at different liquid and gas superficial velocities and pipe diameters and the influence of these parameters on the gas–liquid interface is discussed.

Experimental studies on heat and mass transfer performance of a coiled tube absorber for R134a-DMAC based absorption cooling system

Absorber is an important component in vapor absorption refrigeration system and its performance has greater influence in overall efficiency of absorption machines. Falling film heat and mass transfer in an absorber is greatly influenced by fluid properties, geometry of heat exchanger and its operating parameters. This paper presents on the results of experimental studies on the heat and mass transfer characteristics of a coiled tube falling film absorber, using 1,1,1,2-Tetrafluroethane(R-134a) and N-N Dimethyl Acetamide (DMAC) as working fluids. The effects of film Reynolds number, inlet solution temperature and cooling water temperature on absorber heat load, over all heat transfer coefficient and mass of refrigerant absorbed are presented and discussed. Normalized solution and coolant temperature profiles and refrigerant mass absorbed along the height of absorber are also observed from the experimental results. The optimum over all heat transfer coefficient for R-134a–DMAC solution found to be 726 W/m2K for a film Reynolds number of 350. The R-134a vapour absorption rate is maximum in the normalized coil height of 0.6 to 1.

Prediction of refrigerant absorption and onset of natural convection in lubricant oil

Refrigerant absorption and mixing in lubricant oil are important in the design of refrigeration compressors and refrigeration systems. Experimental work is reported on absorption of R-134a vapour through the top interface of an initially stagnant layer of pure lubricant oil. Since the liquid refrigerant is heavier than the oil, mixing is enhanced due to natural mass convection. In the present paper, the behaviour of the liquid temperature during absorption is described based on measurements carried out in a test rig consisting of a transparent 70 mm ID, 150 mm long, vertical glass tube through which absorption can be directly observed. Transient liquid temperatures were measured at three different heights in the test section (two in the vapour, one in the liquid). The experimental work is complemented by a theoretical analysis of the critical time for the onset of mass transfer induced Rayleigh instability. The model is based on a critical mass transfer Rayleigh number criterion widely reported in the literature and takes into account the variation of physical properties in the liquid layer. The critical time for the onset of natural mass convection increases with decreasing system pressure as a result of a lower equilibrium concentration at the vapour–liquid interface.

Film condensation under normal and microgravity: Effect of channel shape

This paper presents a theoretical model to predict time dependent film condensation heat transfer in 2-dimensional geometries for different channel shapes. Surface tensions, curvature induced pressures, gravitational forces and shear stresses at liquid-wall interfaces are taken into account. Local and mean heat transfer coefficients, liquid film thickness distribution and cross sectional void fraction are evaluated as a function of time. The predicted results for condensation of R134a vapor in a circular channel (1mm diameter), a square channel (1mm side), and an equilateral triangular channel (1mm side) are presented.