Refrigerant R134a vaporisation heat transfer and pressure drop inside a small brazed plate heat exchanger

This paper presents the experimental heat transfer coefficients and pressure drop measured during refrigerant R134a vaporisation inside a small brazed plate heat exchanger (BPHE): the effects of heat flux, refrigerant mass flux, saturation temperature and outlet conditions are investigated. The BPHE tested consists of 10 plates, 72 mm in width and 310 mm in length, which present a macro-scale herringbone corrugation with an inclination angle of 65° and corrugation amplitude of 2 mm.The experimental results are reported in terms of refrigerant side heat transfer coefficients and frictional pressure drop. The heat transfer coefficients show great sensitivity both to heat flux and outlet conditions and weak sensitivity to saturation temperature. The frictional pressure drop shows a linear dependence on the kinetic energy per unit volume of the refrigerant flow.The experimental heat transfer coefficients are also compared with two well-known correlations for nucleate pool boiling and a correlation for frictional pressure drop is proposed.     

CO2 and R410A flow boiling heat transfer, pressure drop, and flow pattern at low temperatures in a horizontal smooth tube

Flow boiling heat transfer coefficient, pressure drop, and flow pattern are investigated in the horizontal smooth tube of 6.1 mm inner diameter for CO₂, R410A, and R22. Flow boiling heat transfer coefficients are measured at the constant wall temperature conditions, while pressure drop measurement and flow visualization are carried out at adiabatic conditions. This research is performed at evaporation temperatures of −15 and −30 °C, mass flux from 100 to 400 kg m⁻² s⁻¹, and heat flux from 5 to 15 kW m⁻² for vapor qualities ranging from 0.1 to 0.8. The measured R410A heat transfer coefficients are compared to other published data. The comparison of heat transfer coefficients for CO₂, R410A, and R22 is presented at various heat fluxes, mass fluxes, and evaporation temperatures. The difference of coefficients for each refrigerant is explained with the Gungor and Winterton [K.E. Gungor, R.H.S. Winterton, A general correlation for flow boiling in tubes and annuli, Int. J. Heat Mass Transfer 29 (1986) 351–358] correlation based on the thermophysical properties of refrigerants. The Wattelet et al. [J.P. Wattelet, J.C. Chato, B.R. Christoffersen, J.A. Gaibel, M. Ponchner, P.J. Kenny, R.L. Shimon, T.C. Villaneuva, N.L. Rhines, K.A. Sweeney, D.G. Allen, T.T. Heshberger, Heat Transfer Flow Regimes of Refrigerants in a Horizontal-tube Evaporator, ACRC TR-55, University of Illinois at Urbana-Champaign, 1994], and Gungor and Winterton [K.E. Gungor, R.H.S. Winterton, A general correlation for flow boiling in tubes and annuli, Int. J. Heat Mass Transfer 29 (1986) 351–358] correlations give the best agreement with the measured heat transfer coefficients for CO₂ and R410A. Pressure drop for CO₂, R410A, and R22 at various mass fluxes, evaporation temperatures and qualities is presented in this paper. The Müller-Steinhagen and Heck [H. Müller-Steinhagen, K. Heck, A simple friction pressure drop correlation for two-phase flow in pipes, Chem. Eng. Process. 20 (1986) 297–308], and Friedel [L. Friedel, Improved friction pressure correlations for horizontal and vertical two-phase pipe flow, in: The European Two-Phase Flow Group Meeting, Ispra, Italy, 1979 (paper E2)] correlation can predict most of the measured pressure drop within the range of ±30%. The relation between pressure drop and properties for each refrigerant is described by applying the Müller-Steinhagen and Heck correlation. The observed two-phase flow patterns for CO₂ and R410A are presented and compared with flow pattern maps. Most of the flow patterns can be determined by the Weisman et al. [J. Weisman, D. Duncan, J. Gibson, T. Crawford, Effect of fluid properties and pipe diameter on two-phase flow patterns in horizontal lines, Int. J. Multiphase Flow 5 (1979) 437–462] flow pattern map.     

Refrigerant R134a condensation heat transfer and pressure drop inside a small brazed plate heat exchanger

This paper presents the experimental tests on HFC-134a condensation inside a small brazed plate heat exchanger: the effects of refrigerant mass flux, saturation temperature and vapour super-heating are investigated.A transition point between gravity controlled and forced convection condensation has been found for a refrigerant mass flux around 20 kg/m² s. For refrigerant mass flux lower than 20 kg/m² s, the saturated vapour heat transfer coefficients are not dependent on mass flux and are well predicted by the Nusselt [Nusselt, W., 1916. Die oberflachenkondensation des wasserdampfes. Z. Ver. Dt. Ing. 60, 541–546, 569–575] analysis for vertical surface. For refrigerant mass flux higher than 20 kg/m² s, the saturated vapour heat transfer coefficients depend on mass flux and are well predicted by the Akers et al. [Akers, W.W., Deans, H.A., Crosser, O.K., 1959. Condensing heat transfer within horizontal tubes. Chem. Eng. Prog. Symp. Ser. 55, 171–176] equation. In the forced convection condensation region, the heat transfer coefficients show a 30% increase for a doubling of the refrigerant mass flux. The condensation heat transfer coefficients of super-heated vapour are 8–10% higher than those of saturated vapour and are well predicted by the Webb [Webb, R.L., 1998. Convective condensation of superheated vapour. ASME J. Heat Transfer 120, 418–421] model. The heat transfer coefficients show weak sensitivity to saturation temperature. The frictional pressure drop shows a linear dependence on the kinetic energy per unit volume of the refrigerant flow and therefore a quadratic dependence on the refrigerant mass flux.     

CO2 flow condensation heat transfer and pressure drop in multi-port microchannels at low temperatures

CO₂ flow condensation heat transfer coefficients and pressure drop are investigated for 0.89 mm microchannels at horizontal flow conditions. They were measured at saturation temperatures of −15 and −25 °C, mass fluxes from 200 to 800 kg m⁻² s⁻¹, and wall subcooling temperatures from 2 to 4 °C. Flow patterns for experimental conditions were predicted by two flow pattern maps, and it could be predicted that annular flow patterns could exist in most of flow conditions except low mass flux and low vapor quality conditions. Measured heat transfer coefficients increased with the increase of mass fluxes and vapor qualities, whereas they were almost independent of wall subcooling temperature changes. Several correlations could predict heat transfer coefficients within acceptable error range, and from this comparison, it could be inferred that the flow condensation mechanism in 0.89 mm channels should be similar to that in large tubes. CO₂ two-phase pressure drop, measured in adiabatic conditions, increased with the increase of mass flux and vapor quality, and it decreased with the increase of saturation temperature. By comparing measured pressure drop with calculated values, it was shown that several correlations could predict the measured values relatively well.     

Evaporation of R407C and R410A in a horizontal herringbone microfin tube: heat transfer and pressure drop

Based on experimental data for R134a, the present work deals with the development of a prediction method for heat transfer in herringbone microfin tubes. As is shown in earlier works, heat transfer coefficients for the investigated herringbone microfin tube tend to peak at lower vapour qualities than in helical microfin tubes. Correlations developed for other tube types fail to describe this behaviour. A hypothesis that the position of the peak is related to the point where the average film thickness becomes smaller than the fin height is tested and found to be consistent with observed behaviour. The proposed method accounts for this hypothesis and incorporates the well-known Steiner and Taborek correlation for the calculation of flow boiling heat transfer coefficients. The correlation is modified by introducing a surface enhancement factor and adjusting the two-phase multiplier. Experimental data for R134a are predicted with an average residual of 1.5% and a standard deviation of 21%. Tested against experimental data for mixtures R410A and R407C, the proposed method overpredicts experimental data by around 60%. An alternative adjustment of the two-phase multiplier, in order to better predict mixture data, is discussed.     

Evaporation heat transfer and pressure drop of ammonia in a mixed configuration chevron plate heat exchanger

Ammonia is a naturally occurring environment friendly refrigerant with attractive thermo-physical properties. Experimental investigation of heat transfer and pressure drop during steady state evaporation of ammonia in a commercial plate heat exchanger has been carried out for an un-symmetric 30°/60° chevron plate configuration. Experiments were conducted for saturation temperatures ranging from −25 °C to −2 °C. The heat flux was varied between 21 kW m⁻² and 44 kW m⁻². Experimental results show significant effect of saturation temperature, heat flux and exit vapor quality on heat transfer coefficient and pressure drop. Current mixed plate configuration data are compared with previous studies on the same heat exchanger with symmetric plate configurations. This comparison highlighted importance of optimization in selection of the heat exchangers. Correlations for two phase Nusselt number and friction factor for each chevron plate configuration considered are developed. A Nusselt number correlation generalized for a range of chevron angles is also proposed.     

Evaporating heat transfer of R22 and R410A in horizontal smooth and microfin tubes

An experimental investigation of condensation heat transfer in 9.52 mm O.D. horizontal copper tubes was conducted using R22 and R410A. The test rig had a straight, horizontal test section with an active length of 0.92 m and was cooled by the heat transfer fluid (cold water) circulated in a surrounding annulus. Constant heat flux of 11.0 kW/m² was maintained throughout the experiment and refrigerant quality varied from 0.9 to 0.1. The condensation test results at 45 °C were reported for 40–80 kg/h mass flow rate. The local and average condensation coefficients for seven microfin tubes were presented compared to those for a smooth tube. The average condensation coefficients of R22 and R410A for the microfin tubes were 1.7–3.19 and 1.7–2.94 times larger than those in smooth tube, respectively.     

An experimental study on heat transfer and pressure drop characteristics of carbon dioxide during gas cooling process in a horizontal tube

The heat transfer coefficient and pressure drop during gas cooling process of CO₂ (R744) in a horizontal tube were investigated experimentally. The experiments are conducted without oil in the refrigerant loop. The main components of the refrigerant loop are a receiver, a variable-speed pump, a mass flow meter, a pre-heater and a gas cooler (test section). The water loop consists of a variable speed pump, an isothermal tank, and a flow meter. The refrigerant, circulated by the variable-speed pump, condenses in the inner tube while water flows in the annulus. The gas cooler of tube diameter is 6000 mm in length, and it is divided into 12 subsections.The pressure drop of CO₂ in the gas cooler shows a relatively good agreement with those predicted by Blasius's correlation. The local heat transfer coefficient of CO₂ agrees well with the correlation by Bringer–Smith. However, at the region near Pseudo-critical temperature, the experiments indicate higher values than the Bringer–Smith correlation. Based on the experimental data presented in this paper, a new correlation to predict the heat transfer coefficient of supercritical CO₂ during in-tube cooling has been developed. The majority of the experimental values are within 18% of the values predicted by the new correlation.     

Condensation heat transfer and pressure drop in flattened microfin tubes having different aspect ratios

In this study, condensation heat transfer coefficients and pressure drops of R-410A are obtained in flattened microfin tubes made from 7.0 mm O.D. round microfin tubes. The test range covers saturation temperature 45 °C, mass flux 100–400 kg m⁻² s⁻¹ and quality 0.2–0.8. Results show that the effect of aspect ratio on condensation heat transfer coefficient is dependent on the flow pattern. For annular flow, the heat transfer coefficient increases as aspect ratio increases. For stratified flow, however, the heat transfer coefficient decreases as aspect ratio increases. The pressure drop always increases as aspect ratio increases. Possible reasoning is provided based on the estimated flow pattern in flat microfin tubes. Comparison with existing round microfin tube correlations is made.     

Forced convective boiling heat transfer of R-410A in horizontal minichannels

Convective boiling heat transfer experiments were performed in horizontal minichannels with binary mixture refrigerant, R-410A. The test section is made of stainless steel tubes with inner diameters of 1.5 mm and 3.0 mm and with lengths of 1500 mm and 3000 mm, respectively, and is uniformly heated by applying electric current directly to the tubes. Local heat transfer coefficients were obtained for a heat flux range of 10–30 kW m⁻², a mass flux range of 300–600 kg m⁻² s⁻¹, and quality ranges of up to 1.0. The experimental results were mapped on Wang et al.'s (C.C. Wang, C.S. Chiang, D.C. Lu, Visual observation of two-phase flow pattern of R-22, R-134a, and R-407C in a 6.5-mm smooth tube, Experimental, Thermal and Fluid Science 15 (1997) 395–405) and Wojtan et al.'s (L. Wojtan, T. Ursenbacher, J.R. Thome, Investigation of flow boiling in horizontal tubes: part I – a new diabatic two-phase flow pattern map, International Journal of Heat and Mass Transfer 48 (2005) 2955–2969) flow pattern maps to observe the flow regimes. Laminar flow appears in flow with minichannels. A new boiling heat transfer coefficient correlation based on the superposition model for R-410A was developed with 11.20% mean deviation; it showed a good agreement between the measured data and the calculated heat transfer coefficients.     

Evaporation heat transfer characteristics of R-410A in 7 and 9.52 mm smooth/micro-fin tubes

In-tube evaporation heat transfer characteristics of R-410A were experimentally investigated and analyzed as a function of evaporating temperature, mass flux, heat flux, and tube geometry. Evaporation heat transfer coefficients and pressure drops were measured for 3.0 m long smooth and micro-fin tubes with outer diameters of 9.52 and 7.0 mm, respectively. The test matrix in the present study included measurements for evaporation over a refrigerant mass flux range of 70–211 kg/m²s, a heat flux range of 5–15 kW/m² and an evaporating temperature range of −15 to 5. The objective of this study is to evaluate the heat transfer enhancement of the micro-fin tube with R-410A as a function of mass flux, heat flux, evaporating temperature and tube diameter.     

Heat transfer and pressure drop in a plate-type evaporator

A plate-type evaporator, working with natural refrigerant circulation, has been investigated both experimentally and theoretically. Motivated by the phase-out of ozone-depleting substances, HCFC22 was compared to HFC134a and two zeotropic refrigerant mixtures. The effect of different separator liquid levels, i.e. refrigerant flows, and its influence on heat transfer was also studied. The investigated plate-type evaporator consists of thirteen vertical flow channels and its size is 3.0 m × 0.5 m. The heat source for the evaporator is a falling water film on the outside of the plate. Experimental studies have been carried out using a test facility that enabled detailed measurements of heat transfer and pressure drop. Experiments were compared to results from a calculation method that simultaneously calculates heat transfer and pressure drop in a variable number of steps along the evaporator. The calculation method is based on a pressure drop correlation proposed by the VDI-Wärmeatlas and a heat transfer correlation for vertical tubes proposed by Steiner and Taborek. For different evaporator duties, heat transfer was over predicted by 12% for pure fluids by 15% for mixtures. Calculated pressure drops were well within ±5% of the measured values. Changes in heat transfer due to different flows were closely predicted by the proposed calculation method.     

NH3 in-tube condensation heat transfer and pressure drop in a smooth tube

This paper presents an overview of the issues and new results for in-tube condensation of ammonia in horizontal round tubes. A new empirical correlation is presented based on measured NH₃ in-tube condensation heat transfer and pressure drop by Komandiwirya et al. [Komandiwirya, H.B., Hrnjak, P.S., Newell, T.A., 2005. An experimental investigation of pressure drop and heat transfer in an in-tube condensation system of ammonia with and without miscible oil in smooth and enhanced tubes. ACRC CR-54, University of Illinois at Urbana-Champaign] in an 8.1 mm aluminum tube at a saturation temperature of 35 °C, and for a mass flux range of 20–270 kg m⁻² s⁻¹. Most correlations overpredict these measured NH₃ heat transfer coefficients, up to 300%. The reasons are attributed to difference in thermophysical properties of ammonia compared to other refrigerants used in generation and validation of the correlations. Based on the conventional correlations, thermophysical properties of ammonia, and measured heat transfer coefficients, a new correlation was developed which can predict most of the measured values within ±20%. Measured NH₃ pressure drop is shown and discussed. Two separated flow models are shown to predict the pressure drop relatively well at pressure drop higher than 1 kPa m⁻¹, while a homogeneous model yields acceptable values at pressure drop less than 1 kPa m⁻¹. The pressure drop mechanism and prediction accuracy are explained though the use of flow patterns.     

Characteristics of evaporative heat transfer and pressure drop of carbon dioxide and correlation development

Carbon dioxide among natural refrigerants has gained considerable attention as an alternative refrigerant due to its excellent thermophysical properties. In this study, transcritical refrigeration cycle using carbon dioxide is of great interest, and the evaporation process is investigated by experiment and analysis. This paper presents the measured heat transfer coefficients and pressure drop during evaporation process of carbon dioxide in a horizontal smooth tube. The test section was made of a seamless stainless steel tube with the inner diameter of 7.53 mm, and length of 5 m. Heat is provided by a direct heating method to the test section. Experiments were conducted at saturation temperatures of −4 to 20 °C, heat fluxes of 12 to 20 kWm⁻² and mass fluxes of 200 to 530 kgm⁻² s⁻¹. A comparison of different heat transfer correlations applicable to evaporation of carbon dioxide has been made. Based on the experiments for the evaporation heat transfer, useful correlation is developed.     

Experimental studies on the evaporative heat transfer and pressure drop of CO2 in smooth and micro-fin tubes of the diameters of 5 and 9.52 mm

Carbon dioxide among natural refrigerants has gained a considerable attention as an alternative refrigerant due to its excellent thermophysical properties. In-tube evaporation heat transfer characteristics of carbon dioxide were experimentally investigated and analyzed as a function of evaporating temperature, mass flux, heat flux and tube geometry. Heat transfer coefficient data during evaporation process of carbon dioxide were measured for 5 m long smooth and micro-fin tubes with outer diameters of 5 and 9.52 mm. The tests were conducted at mass fluxes of from 212 to 656 kg m⁻² s⁻¹, saturation temperatures of from 0 to 20 °C and heat fluxes of from 6 to 20 kW m⁻². The difference of heat transfer characteristics between smooth and micro-fin tubes and the effect of mass flux, heat flux, and evaporation temperature on enhancement factor (EF) and penalty factor (PF) were presented. Average evaporation heat transfer coefficients for a micro-fin tube were approximately 150–200% for 9.52 mm OD tube and 170–210% for 5 mm OD tube higher than those for the smooth tube at the same test conditions. The effect of pressure drop expressed by measured penalty factor of 1.2–1.35 was smaller than that of heat transfer enhancement.     

Experimental study of turbulent single-phase flow and heat transfer inside a micro-finned tube

Single-phase heat transfer and pressure drop characteristics of a commercially available internally micro-finned tube with a nominal outside diameter of 7.94 mm were studied. Experiments were conducted in a double pipe heat exchanger with water as the cooling as well as the heating fluid for six sets of runs. The pressure drop data were collected under isothermal conditions. Data were taken for turbulent flow with 3300 ≤ Re ≤ 22,500 and 2.9 ≤ Pr ≤ 4.7. The heat transfer data were correlated by a Dittus–Boelter type correlation, while the pressure drop data were correlated by a Blasius type correlation. The correlation predicted values for both the Nusselt number and the friction factors were compared with other studies. It was found that the Nusselt numbers obtained from the present correlation fall in the middle region between the Copetti et al. and the Gnielinski smooth tube correlation predicted Nusselt number values. For pressure drop results, the present correlation predicted friction factors values were nearly double that of the Blasius smooth tube correlation predicted friction factors. It was also found that the rough tube Gnielinski and Haaland correlations can be used as a good approximation to predict the finned tube Nusselt number and ffriction factor, respectively, in the tested Reynolds number range.     

Measurement and prediction of evaporator shell-side pressure drop

The pressure drop across a heat exchanger is an important parameter, along with the heat transfer capacity. In fact, the operating cost throughout the life of the exchanger depends on the pressure losses. Therefore, it is important to be able to predict pressure drop accurately as it is to predict heat transfer.A new data set of shell-side pressure drop measurements taken during isothermal flow of brines in shell and tube evaporators was collected in the Alfa Laval laboratory. It covers several different configurations of industrial shell and tube evaporators and a wide range of operating conditions, with cross flow Reynolds number ranging from 170 to 33,000.The database is compared against two predictive procedures available in the literature for computing shell-side pressure drop, showing that no method is accurate enough for design purpose.As a further step, a new suggested procedure is presented, which extends the Wills and Johnston [Wills MJN, Johnston D. A new and accurate hand calculation method for shellside pressure drop and flow distribution. 22nd National Heat Transfer Conference, HTD N. 36. New York: ASME; 1984, p. 67–79] method to the low Reynolds number range and improve its capability to predict experimental data.     

Liquid side heat transfer and pressure drop in finned-tube cooling-coils operated with secondary refrigerants

In this study full-scale experiments with two different conventional cooling-coils aimed for display cabinets were performed. Heat transfer and pressure drop on the liquid side for three different single phase secondary refrigerants were studied and compared to predictions by existing correlations. Predominantly, the laminar flow regime was studied. The results show that when predicting the heat transfer performance on the liquid side of a cooling-coil the Gnielinski correlation for thermally developing flow and uniform wall temperature boundary conditions (T) leads to good agreement for 0.0014 < x^* < 0.017 if 50 < Re < 1700, assuming a new entrance length is formed after each U-bend. In addition, these entrance lengths must also be accounted for, when predicting the pressure drop on the liquid side of the cooling-coil. The uncertainty of measurement can be a problem in this type of investigations and this has been taken into consideration when analysing the results.     

Experimental study on the evaporative heat transfer and pressure drop of CO2 flowing upward in vertical smooth and micro-fin tubes with the diameter of 5 mm

Because of the ozone layer depletion and global warming, new alternative refrigerants are being developed. In this study, evaporation heat transfer characteristic and pressure drop of carbon dioxide flowing upward in vertical smooth and micro-fin tubes were investigated by experiment with regard to evaporating temperature, mass flux and heat flux. The vertical smooth and micro-fin tubes with outer diameter (OD) of 5 mm and length of 1.44 m were selected as a test section to measure the evaporative heat transfer coefficient. The tests were conducted at mass fluxes from 212 to 530 kg/(m² s), saturation temperatures from −5 to 20 °C and heat fluxes from 15 to 45 kW/m², where the test section was heated by a direct heating method. The differences of heat transfer characteristics between the smooth and the micro-fin tubes were analyzed with respect to enhancement factor (EF) and penalty factor (PF). Average evaporation heat transfer coefficients for the micro-fin tube were approximately 111–207% higher than those for the smooth tube at the same test conditions, and PF was increased from 106 to 123%.     

Condensation heat transfer of R410A inside internally grooved horizontal tubes

Heat transfer coefficients were measured for the condensation of R410A and R22 inside internally grooved horizontal tubes. The experiment was performed for five different kinds of internally grooved tubes of about 8.00 mm o.d. the shapes of which were conventional helical grooved and herring-bone grooved ones. To measure the local heat transfer coefficients, the test section was subdivided into 10 small sections having 1 m working length. The ranges tested are as follows: the refrigerant mass velocity was from 130 to 400 kg/(m²/s) for R410A and R22, and the vapour pressure was 2.4l MPa for R410A and 1.53 MPa for R22. The obtained heat transfer data of R410A and R22 indicate that the values of the local heat transfer coefficients of the herring-bone grooved tube are about twice as large as those of helical one. All measured local heat transfer coefficients of condensation were compared with the predicted values from previous correlations proposed by other researchers, and were well correlated with the empirical equation using the frictional coefficients for each tube proposed by the author.