High-Pressure Condensing Refrigerant Flows through Microchannels,Part I: Pressure Drop Models

ABSTRACT

Condensation of high-pressure refrigerants in small-diameter channels over a wide range of reduced pressures approaching the critical point is investigated in this two-part study. In this paper, Part I of the study, a multi-regime pressure drop model for condensing fluids in small-diameter channels is presented. Pressure drop measurements were conducted on condensing R404A in circular channels (inner diameter = 0.508, 1.00, 3.05 mm) over the entire quality range. Saturation temperatures were varied from 30 to 60°C, and mass fluxes from 200 to 800 kg m^?2 s^?1, to evaluate their effect on pressure drop. The saturation temperatures investigated here correspond to reduced pressures between 0.38 and 0.77. The pressure drop models are developed using a microchannel flow regime map and the void fraction models presented by the authors in previous work. The resulting model predicts 85.5% of the data within ±25%. Part II of the study presents a corresponding heat transfer model.     

Experimental study of horizontal flattened tubes performance on condensation of R600a vapor

An empirical setup has been established to study heat transfer and pressure drop characteristics during condensation of R600a, a hydrocarbon refrigerant, in a horizontal plain tube and different flattened channels. Round copper tubes of 8.7 mm I.D. were deformed into flattened channels with different interior heights of 6.7 mm, 5.2 mm and 3.1 mm as test sections. The test conditions include heat flux of 17 kw/m², mass velocity in the range of 154.8–265.4 kg/m²s and vapor quality variation from approximately 10% to 80%. Results indicate that flattening the tubes causes significant enhancement of heat transfer coefficient which is also accompanied by simultaneous augmentation in flow pressure drop. Therefore, the overall performance of the flattened tubes with respect to heat transfer enhancement considering the pressure drop penalty is analyzed. It is concluded that the flattened tube with 5.2 mm inner height tube has the best overall performance. Due to the failure of pre-existing correlations for round tube condensation heat transfer, a new correlation is proposed which predicts 90% of the entire data within ± 17% error.     

Heat Transfer Correlations for Single-Phase Flow,Condensation, and Boiling in Microfin Tubes

Experimental single-phase, condensation and flow boiling heat transfer data from the literature and our previous studies were collected to evaluate existing heat transfer correlations for microfin tubes of different geometries. The Ravigururajan and Bergles correlation modified by using the hydraulic diameter proposed by Li et al. (2012) can predict single-phase heat transfer data relatively well. Among the four reviewed condensation heat transfer correlations, the Yu and Koyama (1998) correlation presents the best prediction. However, all the four condensation correlations are prone to overpredict the carbon dioxide data. For flow boiling in microfin tubes, the general semiempirical correlation developed by Wu et al. (2013), applicable for intermittent and annular flow patterns, is the most reliable predictive method among the five evaluated correlations. It can predict 90% of the overall 754 data points within a ±30% error band, with a mean absolute deviation and a standard deviation equal to 18.2% and 21.9%, respectively, covering pure halogenated refrigerants, near azeotropic refrigerant mixtures, and carbon dioxide with the following applicable range: fin root diameter 2.1 to 14.8 mm, mass flux 100 to 800 kg/m²s, heat flux 4.5 to 59 kW/m², and reduced pressure 0.07 to 0.7.     

Numerical simulation of condensation for R410A at varying saturation temperatures in mini/micro tubes

ABSTRACT

Heat transfer and pressure drop characteristics of condensation for R410A inside horizontal tubes (d_h = 0.25, 1, and 2 mm) at saturation temperatures Tsat = 310, 320, and 330 K are investigated numerically. The results indicate that local heat transfer coefficients and pressure drop gradients increase with increasing mass flux and vapor quality and with decreasing tube diameter and saturation temperature. Liquid film thickness also increases with increasing saturation temperature because of the lower surface tension at higher saturation temperature. When gravity dominates the condensation process, a vortex with its core lying at the bottom of the tube is found in the vapor phase region. For the annular flow regime, stream traces point from the symmetry plan to the liquid–vapor interface, where the vapor phase becomes the liquid phase. Numerical heat transfer coefficients and pressure drop gradients are compared to available empirical correlations. Two new models for heat transfer coefficients and frictional pressure drop gradients are developed based on the numerical work.     

Theoretical model for annular flow condensation in rectangular micro-channels

This study examines the pressure drop and heat transfer characteristics of annular condensation in rectangular micro-channels with three-sided cooling walls. A theoretical control-volume-based model is proposed based on the assumptions of smooth interface between the annular liquid film and vapor core, and uniform film thickness around the channel’s circumference. Mass and momentum conservation are applied to control volumes encompassing the liquid film and the vapor core separately. The model accounts for interfacial suppression of turbulent eddies due to surface tension with the aid of a new eddy diffusivity model specifically tailored to shear-driven turbulent films. The model predictions are compared with experimental pressure drop and heat transfer data for annular condensation of FC-72 along 1 × 1 mm² parallel channels. The condensation is achieved by rejecting heat to a counterflow of water. The data span FC-72 mass velocities of 248–367 kg/m² s, saturation temperatures of 57.8–62.3 °C, qualities of 0.23–1.0, and water mass flow rates of 3–6 g/s. The data are also compared to predictions of previous separated flow mini/micro-channel and macro-channel correlations. While some of the previous correlations do provide good predictions of the average heat transfer coefficient, they fail to capture axial variation of the local heat transfer coefficient along the channel. The new model accurately captures the pressure drop and heat transfer coefficient data in both magnitude and trend, evidenced by mean absolute error values of 3.6% and 9.3%, respectively.     

Update on Condensation Heat Transfer and Pressure Drop inside Minichannels

The present paper reviews published experimental work focusing on condensation flow regimes, heat transfer, and pressure drop in minichannels. New experimental data are available with high (R410A), medium (R134a), and low (R236ea) pressure refrigerants in minichannels of different cross-section geometries and with hydraulic diameters ranging from 0.4 to 3 mm. Because of the influence of flow regimes on heat transfer and pressure drop, a literature review is presented to discuss flow regimes transitions. The available experimental frictional pressure gradients and heat transfer coefficients are compared with semi-empirical and theoretical models developed for conventional channels and models specifically created for minichannels. Starting from the results of the comparison between experimental data and models, the paper will discuss and evaluate the opportunity for a new heat transfer model for condensation in minichannels; the new model attempts to take into account the effect of the entrainment rate of droplets from the liquid film.     

Experimental Study on Subcooled Flow Boiling in Horizontal Microtubes and Effect of Heated Length

ABSTRACT

In this study, subcooled flow boiling was investigated in horizontal microtubes. Experiments were conducted using deionized water as the working fluid over a mass flux range of 4000–7000 kg m^?2s^?1 in microtubes with inner and outer diameters of ～600 and ～900 μm, respectively. Microtubes with lengths of 3, 6, and 12 cm were tested to clarify the effect of heated length on flow boiling heat transfer and pressure drop characteristics. A force analysis related to two-phase flow was conducted to understand the effect of forces on bubble dynamics. Pressure drop and heat transfer data in flow boiling were acquired. Experimental heat flux data were compared with partial boiling heat flux correlations, and good agreements were obtained. Pressure drop was larger in longer microtubes in comparison to shorter ones, while higher heat fluxes were obtained in shorter microtubes at the same wall superheat. Two-phase heat transfer coefficient increased with the microtube length due to lower temperature difference between wall temperature and bulk fluid temperature in longer microtubes. Higher heat fluxes achieved in shorter microtubes at the same wall superheat imply higher critical heat fluxes in shorter microtubes.     

Numerical study of nanofluids condensation heat transfer in a square microchannel

ABSTRACT

This paper presents a numerical study of nanofluids condensation heat transfer inside a single horizontal smooth square tube. The numerical results are compared with the previous experimental predictions. The numerical results show that the heat transfer coefficient could be improved within 20% by increasing the volume fraction of Cu nanoparticle by 5% or by increasing the mass flux from 80 to 110 kg/m² s. Reducing the hydraulic diameter of the microchannel from 200 to 160 µm leads to an increase in the condensation average heat transfer coefficient by 10%. A new correlation estimating the Nusselt number for the condensation of nanofluids or pure vapor is proposed. It predicts average condensation heat transfer with a good agreement with those computed.     

Condensation Heat Transfer and Pressure Gradient Inside Multiport Minichannels

Abstract

In this paper, the experimental heat transfer coefficients measured during condensation of R134a and R410A inside multiport minichannels are presented. The frictional pressure gradient was also measured during adiabatic two-phase flow. The need for experimental research on condensation inside multiport minichannels comes from the wide use of those channels in automotive air-conditioners. The perspective for the adoption of similar channels in the residential air conditioning applications also calls for experimental research on new high pressure refrigerants, such as R410A.

Experimental data are compared against models to show the accuracy of the models in the prediction of heat transfer coefficients and pressure drop inside minichannels.     

Interactions Between Parallel Unevenly Heated Minichannels During Flow Boiling of R134A

Transient pressure drop of individual channels during flow boiling of R134a in four 0.54 mm square parallel minichannels was experimentally studied in this work. The design of the test section enabled the experimenter to control and to vary heat flux independently in each channel in the range from 3.82 to 18.66 kW/m² at five different overall flow rates from 86 to 430 kg/m²-s. Flow rate fluctuation in parallel channels due to the formation of bubbles under the nonuniform heat flux conditions caused significant oscillations in local pressure drop. Statistical analysis indicated that the pressure drop signal was normally distributed when boiling was stable with no incoming flow disturbance. Pressure drop distribution was highly skewed and multimodal when significant evaporation rate at low mass fluxes led to rapid annular flow formation, reducing the free flow of incoming fluid. Cross-correlation analysis revealed a strong interaction between minichannels having the highest heat flux difference among the set of channels. The least heated channel was more sensitive to the fluctuations in other channels. Cross-correlation between the most heated channel and the adiabatic one was estimated to be 39% when the total flow rate was the lowest, 86 kg/m²-s. The power of the relationship between channels dropped significantly as the flow rate increased. Less than 5% of data points could be considered cross-correlated at the highest flow rate of 430 kg/m²-s. Increasing the two-phase pressure drop across each channel caused higher resistance to the incoming disturbances and led to less interchannel interaction. This study of the channels interaction in a system of parallel, nonuniformly heated minichannels can be used as a tool to identify and quantify instabilities and reversed flow conditions.     

Flow Boiling Heat Transfer of Refrigerant R-134a in Copper Microchannel Heat Sink

In this paper we present experimental data on heat transfer and pressure drop characteristics at flow boiling of refrigerant R-134a in a horizontal microchannel heat sink. The primary objective of this study was to experimentally establish how the local heat transfer coefficient and pressure drop correlate with the heat flux, mass flux, and vapor quality. The copper microchannel heat sink contains 21 microchannels with 335 × 930 μm² cross section. The microchannel plate and heating block were divided by the partition wall for the local heat flux measurements. Distribution of local heat transfer coefficients along the length and width of the microchannel plate was measured in the range of external heat fluxes from 50 to 500 kW/m²; the mass flux varied within 200–600 kg/m²-s, and pressure varied within 6–16 bar. The obvious impact of heat flux on the magnitude of heat transfer coefficient was observed. It showed that nucleate boiling is the dominant mechanism for heat transfer. A new model of flow boiling heat transfer, considering nucleate boiling suppression and liquid film evaporation, was proposed and verified experimentally in this paper.     

Experiments on the characteristics of evaporation of R410A in brazed plate heat exchangers with different geometric configurations

Experiments on the evaporative heat transfer and pressure drop in the brazed plate heat exchangers were performed with refrigerants R410A and R22. The plate heat exchangers with different 45°, 35°, and 20° chevron angles are used. Varying the mass flux of refrigerant (13–34 kg/m² s), the evaporating temperature (5, 10 and 15 °C), the vapor quality (0.9–0.15) and heat flux (2.5, 5.5 and 8.5 kW/m²), the evaporation heat transfer coefficients and pressure drops were measured. The heat transfer coefficient increases with increasing vapor quality and decreasing evaporating temperature at a given mass flux in all plate heat exchangers. The pressure drop increases with increasing mass flux and quality and with decreasing evaporating temperature and chevron angle. It is found that the heat transfer coefficients of R410A are larger than those of R22 and the pressure drops of R410A are less than those of R22. The empirical correlations of Nusselt number and friction factor are suggested for the tested PHEs. The deviations between correlations and experimental data are within ±25% for Nusselt number and ±15% for friction factor.     

Determination of annular condensation heat transfer coefficient of steam in microchannels with trapezoidal cross sections

Experiments have been carried out to determine annular condensation heat transfer coefficient of steam in two silicon microchannels having trapezoidal cross sections with the same aspect ratio of 3.15 at 54 < G < 559 kg/m² s under 3-side cooling conditions. A semi-analytical method, based on turbulent flow boundary layer theory of liquid film with correlations of pressure drop and void fraction valid for microchannels, is used to derive the annular local condensation heat transfer coefficients. The predicted values based on the semi-analytical model are found within ±20% of 423 data points. It is shown that the annular condensation heat transfer coefficient in a microchannel increases with mass flux and quality and decreases with the hydraulic diameter.     

Thermal Characteristics of a Chip-Scale Two-Phase Microgap Cooler

Two-phase heat transfer and pressure drop results for a chip-scale, uniformly heated, microgap channel, with nominal gap heights of 100, 200, and 500 μm and using HFE-7100 and FC-87 as the working fluids, are reported. Average heat transfer coefficients in the range of 5 to 30 kW/m²-K were observed, for exit qualities up to 60%. Local heat transfer coefficients, obtained through an inverse computational technique, are found to vary strongly with thermodynamic quality and to fall within ±30% of the predictions of the venerable Chen correlation.     

Thermal and flow characteristics of the condensation of R404A refrigerant in pipe minichannels

This article presents the results of experimental investigations of heat exchange and pressure drop during the condensation of R404A refrigerant in stainless steel pipe minichannels with internal diameters of 1.4–3.30 mm. A review is provided of the present state of knowledge concerning the condensation of this refrigerant in conventional channels and in small-diameter channels. It is emphasized that there are few prior publications concerning this issue. The test setup is described as well as the results of the experimental tests. We discuss the dependence of the heat transfer coefficient and the pressure drop of the R404A refrigerant on both minichannel diameter and process parameters. The pressure drop during the condensation of this refrigerant is satisfactorily described by the Friedel and Garimella correlations. Based on the experimental tests, we propose a new correlation for the calculation of the local heat transfer coefficient. The values calculated from this correlation were in agreement with the experimental results to within ±20%.     

Boiling Heat Transfer and Pressure Drop of a Refrigerant R32 Flowing in a Small Horizontal Tube

In this study, experiments were performed to examine characteristics of flow boiling heat transfer and pressure drop of a low global warming potential refrigerant R32 flowing in a horizontal copper circular tube with 1.0 mm inside diameter for the development of a high-performance heat exchanger using small-diameter tubes or minichannels for air conditioning systems. Axially local heat transfer coefficients were measured in the range of mass fluxes from 30 to 400 kg/(m²·s), qualities from 0.05 to 1.0, and heat fluxes from 2 to 24 kW/m² at the saturation temperature of 10°C. Pressure drops were also measured in the rage of mass fluxes from 30 to 400 kg/(m²·s) and qualities from 0.05 to 0.9 at the saturation temperature of 10°C under adiabatic condition. In addition, two-phase flow patterns were observed through a sight glass fixed at the tube exit with a digital camera. The characteristics of boiling heat transfer and pressure drop were clarified based on the measurements and the comparison with data of R410A obtained previously. Also, measured heat transfer coefficients were compared with two existing correlations.     

Flow Boiling Heat Transfer Characteristics of R600a in Multiport Minichannel

ABSTRACT

Evaporators of small and medium refrigeration systems, as in commercial and automobile air conditioning applications, are being studied to develop more compact and lighter equipment, that reaches good thermal performance and reliability, with low pressure drop. In this way, evaporators are being designed with small channels and materials like aluminum. Moreover, different refrigerants are being tested to substitute for hydrofluorocarbon (HFC) refrigerants, with different operational temperatures and pressures. Some of them, like hydrocarbons, although they present advantages with respect to their thermodynamic and transport properties, should be used with small charge in the system due to their flammability. This work presents the results of an experimental study to characterize the flow boiling of the refrigerant R600a (isobutane) in a multiport aluminum extruded tube with 7 parallel minichannels of 1.47 mm hydraulic diameter. The effects of mass velocity, heat flux, and vapor quality on heat transfer were investigated for constant saturation temperature and pressure. Heat fluxes in the range from 5 to 30 kW m^?2, mass velocities set to discrete values in the range of 50 to 200 kg m^?2 s^?1, and saturation temperature of 20°C were considered. It was verified a significant effect of heat flux. Moreover, some images of flow patterns, in different conditions, are presented, and the main patterns identified were slug, intermittent, and annular.     

An Experimental Study of Carbon Dioxide Condensation in Mini Channels

In this study, the local characteristics of pressure drop and heat transfer were investigated experimentally for carbon dioxide condensation in a multi-port extruded aluminum test section, which had 10 circular channels each with 1.31 mm inner diameter. The CO2 was cooled with cooling water flow inside the copper blocks that were attached at both sides of the test section. The temperatures at the outer surface of the test section were measured with 24 K-type thermocouples embedded in the upper and lower surfaces along the length. Local heat fluxes were measured with 12 heat flux sensors to estimate the local enthalpies, temperatures and heat transfer coefficients. Bulk mean temperatures of CO2 at the inlet and outlet of the test section were measured with 2 K-type thermocouples. The measurements were performed for the pressure ranged from 6.48 to 7.3 MPa, inlet temperature of CO2 from 21.63 to 31.33℃, heat flux from 1.10 to 8.12 kW/m2, mass velocity from 123.2 to 315.2 kg/m2s, and vapor quality from 0 to 1. The results indicate that pressure drop is very small along the test section, heat transfer coefficient in the two-phase region is higher than that in the single-phase, and mass velocity has important effect on condensation heat transfer characteristics. In addition, experimental data were compared with previous correlations and large discrepancies were observed.     

Ammonia two-phase flow in a horizontal smooth tube: Flow pattern observations,diabatic and adiabatic frictional pressure drops and assessment of prediction methods

The present study illustrates new experimental two-phase flow pattern observations together with diabatic boiling and adiabatic two-phase frictional pressure drop results for ammonia (R717) flowing inside a 14-mm internal diameter, smooth horizontal stainless steel tube. The flow pattern observations were made for mass velocities of 50, 100 and 160 kg s^?1 m^?2 and saturation temperatures of ?14, ?2 and 12 °C for vapor qualities ranging from 0.05 to 0.6. The flow patterns observed during the study included: stratified-wavy, slug-stratified-wavy, slug, intermittent and annular. For all the experimental conditions, the flow structure observations were compared against the predictions of the flow pattern map model of Wojtan et al. [L. Wojtan, T. Ursenbacher, J.R. Thome, Investigation of flow boiling in horizontal tubes: part I – a new diabatic two-phase flow pattern map, Int. J. Heat Mass Transfer 48 (2005) 2955–2969] and showed very good correspondence. The frictional pressure drop measurements were obtained for vapor qualities from 0.05 to 0.6, saturation temperatures from ?14 to 14 °C, mass velocities from 50 to 160 kg s^?1 m^?2 and heat fluxes from 12 to 25 kW m^?2. The experimental results show the traditional pressure drop trends: the frictional pressure drop increases with vapor quality and mass velocity. Moreover, the results also show that both diabatic and adiabatic frictional pressure drop values are similar, that is, the boiling process in itself does not affect the frictional pressure drop. The correlations of Friedel [L. Friedel, Improved friction drop correlations for horizontal and vertical two-phase pipe flow, in: European Two-Phase Flow Group Meeting, paper E2, Ispra, Italy, 1979], Lockhart and Martinelli [R.W. Lockhart, R.C. Martinelli, Proposed correlation of data for isothermal two-phase two-component in pipes, Chem. Eng. Process 45 (1949) 39–48] and Müller-Steinhagen and Heck [H. Müller-Steinhagen, K. Heck, A simple friction pressure correlation for two-phase flow in pipes, Chem. Eng. Process 20 (1986) 297–308] predicted only 54%, 52% and 60% of the experimental data within ±30%, respectively. The correlation of Grönnerud [R. Grönnerud, Investigation of liquid hold-up, flow-resistance and heat transfer in circulation type of evaporators, part iv: two-phase flow resistance in boiling refrigerans, in: Annexe 1972-1, Bull. de l’Inst. Froid, 1979] predicted 93% of the data and the flow pattern based method of Moreno Quibén and Thome [J. Moreno Quibén, J.R. Thome, Flow pattern based two-phase frictional pressure drop model for horizontal tubes. Part II: new phenomenological model, Int. J. Heat Fluid Flow 28 (2007) 1060–1072] predicted more than 97% of the experimental data within the same error band, while the latter method captures almost 89% of the data within ±20%.     

Steam condensing – liquid CO2 boiling heat transfer in a steam condenser for a new heat recovery system

In a new waste heat recovery system, waste heat is recovered from steam condensers through cooling by liquid CO₂ instead of seawater, taking advantage of effective boiling heat transfer performance; the heat is subsequently used for local heat supply. The steam condensing – liquid CO₂ boiling heat transfer performance in a steam condenser with a shell and a helical coil non-fin tube was studied both numerically and experimentally. A heat transfer numerical model was constructed from two models developed for steam condensation and for liquid CO₂ boiling. Experiments were performed to verify the model at a steam pressure range of 3.2–5 kPa and a CO₂ saturation pressure range of 5–6 MPa. Overall heat transfer coefficients obtained from the numerical model agree with the experimental data within ±5%. The numerical estimations show that the boiling local heat transfer coefficient reaches a maximum value of 26 kW/m² K. This value is almost one order higher than that of a conventional water-cooled condenser.