Two-phase flow in high-heat-flux micro-channel heat sink for refrigeration cooling applications: Part II—heat transfer characteristics

This paper is the second of a two-part study concerning two-phase flow and heat transfer characteristics of R134a in a micro-channel heat sink incorporated as an evaporator in a refrigeration cycle. Boiling heat transfer coefficients were measured by controlling heat flux (q″ = 15.9 − 93.8 W/cm²) and vapor quality (x_e = 0.26 − 0.87) over a broad range of mass velocity. While prior studies point to either nucleate boiling or annular film evaporation (convective flow boiling) as dominant heat transfer mechanisms in small channels, the present study shows heat transfer is associated with different mechanisms for low, medium and high qualities. Nucleate boiling occurs only at low qualities (x_e < 0.05) corresponding to very low heat fluxes, and high fluxes produce medium quality (0.05 < x_e < 0.55) or high quality (x_e > 0.55) flows dominated by annular film evaporation. Because of the large differences in heat transfer mechanism between the three quality regions, better predictions are possible by dividing the quality range into smaller ranges corresponding to these flow transitions. A new heat transfer coefficient correlation is recommended which shows excellent predictions for both R134a and water.     

First and second laws analysis of the heat pipe/ejector refrigeration cycle

Integration of the heat pipe with an ejector will result in a compact and high performance system. The concept of the heat pipe/ejector refrigeration cycle is discussed, in this paper. The needed driving capillary forces are firmly established. The basic characteristics of the system, such as entrainment ratio, coefficient of performance, exergy efficiency and thermal efficiency of the system are evaluated. Also, the zero-dimensional constant pressure mixing theory is applied to ejector. In this study, water is used as the working fluid. Whenever the mixed flow is supersonic, a normal shockwave is assumed to occur upstream of diffuser inlet. The simulation results indicate that, the coefficient of performance can reach about 0.30 at T_e = 10 °C, T_c = 30 °C and T_g = 100 °C. Also, the second law efficiency of the heat pipe/ejector refrigeration cycle increases with increasing evaporator temperature and decreasing condenser temperature. It is seen that, the maximum heat pipe cooling capacity obtains for large heat pipe diameters, near the small heat pipe lengths. It has proven that, this refrigeration system can be widely used in many areas, especially in renewable energy utilization such as solar energy.     

Consolidated method to predicting pressure drop and heat transfer coefficient for both subcooled and saturated flow boiling in micro-channel heat sinks

Published studies concerning transport phenomena in micro-channel heat sinks can be divided into those concerning saturated boiling versus those focused on subcooled boiling, with the vast majority related to the former. What has been lacking is a single generalized method to tackle both boiling regimes. The primary objective of the present paper is to construct a consolidated method to predicting transport behavior of micro-channel heat sinks incurring all possible heat transfer regimes. First, a new correlation is developed for subcooled flow boiling pressure drop that accounts for inlet subcooling, micro-channel aspect ratio, and length-to-diameter ratio. This correlation shows excellent predictive capability against subcooled HFE 7100 pressure drop data corresponding to four different micro-channel geometries. Next, a consolidated method is developed for pressure drop that is capable of tackling inlet single-phase liquid, subcooled boiling, saturated boiling, and single-phase vapor regimes as well as inlet contraction and outlet expansion. A similar consolidated method is developed to predict the heat transfer coefficient that is capable of tackling all possible combinations of heat transfer regimes. The new consolidated method is shown to be highly effective at reproducing both data and trends for HFE 7100, water and R134a.     

Electrohydrodynamic enhancement of in-tube convective condensation heat transfer

An experimental investigation of electrohydrodynamic (EHD) augmentation of heat transfer for in-tube condensation of flowing refrigerant HFC-134a has been performed in a horizontal, single-pass, counter-current heat exchanger with a rod electrode placed in the centre of the tube. The effects of varying the mass flux (55 kg/m² s ⩽ G ⩽ 263 kg/m² s), inlet quality (0.2 ⩽ x_in ⩽ 0.83) and the level of applied voltage (0 kV ⩽ V ⩽ 8 kV) are examined. The heat transfer coefficient was enhanced by a factor up to 3.2 times for applied voltage of 8 kV. The pressure drop was increased by a factor 1.5 at the same conditions of the maximum heat transfer enhancement. The improved heat transfer performance and pressure drop penalty are due to flow regime transition from stratified flow to annular flow as has been deduced from the surface temperature profiles along the top and bottom surfaces of the tube.     

An experimental study of heat transfer in oscillating flow through a channel filled with an aluminum foam

An experimental study has been conducted on the heat transfer of oscillating flow through a channel filled with aluminum foam subjected to a constant wall heat flux. The surface temperature distribution on the wall, velocity of flow through porous channel and pressure drop across the test section were measured. The characteristics of pressure drop, the effects of the dimensionless amplitude of displacement and dimensionless frequency of oscillating flow on heat transfer in porous channel were analyzed. The results revealed that the heat transfer in oscillating flow is significantly enhanced by employing porous media in a plate channel. The cycle-averaged local Nusselt number increases with both the kinetic Reynolds number Re_ω and the dimensionless amplitude of flow displacement A₀. The length-averaged Nusselt number is effectively increased by increasing the kinetic Reynolds number from 178 to 874 for A₀ = 3.1-4.1. Based on the experimental data, a correlation equation of the length-averaged Nusselt number with the dimensionless parameters of Re_ω and A₀ is obtained for a porous channel with L/D_h = 3.     

Energy and exergy analyses of an externally fired gas turbine (EFGT) cycle integrated with biomass gasifier for distributed power generation

Biomass based decentralized power generation using externally fired gas turbine (EFGT) can be a technically feasible option. In this work, thermal performance and sizing of such plants have been analyzed at different cycle pressure ratio (r_p = 2−8), turbine inlet temperature (TIT = 1050–1350 K) and the heat exchanger cold end temperature difference (CETD = 200–300 K). It is found that the thermal efficiency of the EFGT plant reaches a maximum at an optimum pressure ratio depending upon the TIT and heat exchanger CETD. For a particular pressure ratio, thermal efficiency increases either with the increase in TIT or with the decrease in heat exchanger CETD. The specific air flow, associated with the size of the plant equipment, decreases with the increase in pressure ratio. This decrease is rapid at the lower end of the pressure ratio (r_p < 4) but levels-off at higher r_p values. An increase in the TIT reduces the specific air flow, while a change in the heat exchanger CETD has no influence on it. Based on this comparison, the performance of a 100 kW EFGT plant has been analyzed for three sets of operating parameters and a trade-off in the operating condition is reached.     

Local turbulent opposing mixed convection heat transfer in inclined flat channel for unstably stratified airflow

Local turbulent mixed convection heat transfer in inclined (from ? = 0° (horizontal position) till ? = 90° (vertical position)) flat channels for opposing flows was investigated for the case when only bottom wall is heated (unstably stratified flow conditions). Wide ranges of airflow parameters are covered: Re = 4 × 10³-6.6 × 10⁴, Gr_q = 4.7 × 10⁷-6.3 × 10¹⁰, pressure p = 0.1; 0.2; 0.4; 0.7; 1.0 MPa. Correlation for calculation of heat transfer in inclined flat channels was suggested for the region without buoyancy instabilities. The experimental data were compared with the recent experimental data for inclined flat channels when upper wall is heated (stably stratified flow conditions).     

Experimental analysis for the determination of the convective heat transfer coefficient by measuring pressure drop directly during annular condensation flow of R134a in a vertical smooth tube

This study investigated the direct relationship between the measured condensation pressure drop and convective heat transfer coefficient of R134a flowing downward inside a vertical smooth copper tube having an inner diameter of 8.1 mm and a length of 500 mm during annular flow. R134a and water were used as working fluids on the tube side and annular side of a double tube heat exchanger, respectively. Condensation experiments were performed at mass fluxes of 260, 300, 340, 400, 456 and 515 kg m⁻² s⁻¹ in the high mass flux region of R134a. The condensing temperatures were around 40 and 50 °C; the heat fluxes were between 10.16 and 66.61 kW m⁻². Paliwoda’s analysis, which focused mainly on the determination of the two-phase flow factor and two-phase length of evaporators and condensers, was adapted to the in-tube condensation phenomena in the test section to determine the condensation heat transfer coefficient, heat flux, two-phase length and pressure drop experimentally by means of a large number of data points obtained under various experimental conditions.     

Experimental study of condensation heat transfer of R-134a flow in corrugated tubes with different inclinations

This experimental study is performed to investigate condensation heat transfer coefficient of R-134a flow inside corrugated tube with different inclinations. Different inclinations of test condenser ranging from − 90^° to + 90^° and various flow mass velocities in the range of 87 to 253 [kg/m²s] are considered in this study. Data analysis showed that change in the tube inclination had a significant effect on condensation heat transfer behavior. At low mass velocities, and low vapor qualities, the highest condensation heat transfer coefficient was obtained for α = + 30^° which was 1.41 times greater than the least one obtained for α = − 90^°. The results also showed that at all mass velocities, the highest average heat transfer coefficients were achieved for α = + 30^°. Based on the experimental results, a new empirical correlation is proposed to predict the condensation heat transfer coefficient of R134a flow in corrugated tubes with different inclinations.     

Experimental study of evaporation heat transfer characteristics of refrigerants R-134a and R-407C in horizontal small tubes

An experiment is carried out here to investigate the characteristics of the evaporation heat transfer for refrigerants R-134a and R-407C flowing in horizontal small tubes having the same inside diameter of 0.83 or 2.0 mm. In the experiment for the 2.0-mm tubes, the refrigerant mass flux G is varied from 200 to 400 kg/m² s, imposed heat flux q from 5 to 15 kW/m², inlet vapor quality x_in from 0.2 to 0.8 and refrigerant saturation temperature T_sat from 5 to 15 °C. While for the 0.83-mm tubes, G is varied from 800 to 1500 kg/m² s with the other parameters varied in the same ranges as those for D_i = 2.0 mm. In the study the effects of the refrigerant vapor quality, mass flux, saturation temperature and imposed heat flux on the measured evaporation heat transfer coefficient h_r are examined in detail. The experimental data clearly show that both the R-134a and R-407C evaporation heat transfer coefficients increase almost linearly and significantly with the vapor quality of the refrigerant, except at low mass flux and high heat flux. Besides, the evaporation heat transfer coefficients also increase substantially with the rises in the imposed heat flux, refrigerant mass flux and saturation temperature. At low R-134a mass flux and high imposed heat flux the evaporation heat transfer coefficient in the smaller tubes (D_i = 0.83 mm) may decline at increasing vapor quality when the quality is high, due to the partial dryout of the refrigerant flow in the smaller tubes at these conditions. We also note that under the same x_in, T_sat, G, q and D_i, refrigerant R-407C has a higher h_r when compared with that for R-134a. Finally, an empirical correlation for the R-134a and R-407C evaporation heat transfer coefficients in the small tubes is proposed.     

Modelling and performance study of a continuous adsorption refrigeration system driven by parabolic trough solar collector

This article suggests a numerical study of a continuous adsorption refrigeration system consisting of two adsorbent beds and powered by parabolic trough solar collector (PTC). Activated carbon as adsorbent and ammonia as refrigerant are selected. A predictive model accounting for heat balance in the solar collector components and instantaneous heat and mass transfer in adsorbent bed is presented. The validity of the theoretical model has been tested by comparison with experimental data of the temperature evolution within the adsorber during isosteric heating phase. A good agreement is obtained. The system performance is assessed in terms of specific cooling power (SCP), refrigeration cycle COP (COP_cycle) and solar coefficient of performance (COP_s), which were evaluated by a cycle simulation computer program. The temperature, pressure and adsorbed mass profiles in the two adsorbers have been shown. The influences of some important operating and design parameters on the system performance have been analyzed.The study has put in evidence the ability of such a system to achieve a promising performance and to overcome the intermittence of the adsorption refrigeration systems driven by solar energy. Under the climatic conditions of daily solar radiation being about 14 MJ per 0.8 m² (17.5 MJ/m²) and operating conditions of evaporating temperature, T_ev = 0 °C, condensing temperature, T_con = 30 °C and heat source temperature of 100 °C, the results indicate that the system could achieve a SCP of the order of 104 W/kg, a refrigeration cycle COP of 0.43, and it could produce a daily useful cooling of 2515 kJ per 0.8 m² of collector area, while its gross solar COP could reach 0.18.     

Enthalpy change (ΔH) and entropy change (ΔS) measurement of CeMn1−xAl1−xNi2x (x=0.00, 0.25, 0.50 and 0.75) hydrides by electrochemical P–C–T curve

The thermodynamic properties of CeMn_1−xAl_1−xNi_2x (x=0.00, 0.25, 0.50 and 0.75) hydrides have been investigated in this paper. With increasing Ni substitution content, the hydrogen concentration (H/M) in CeMn_1−xAl_1−xNi_2x (x=0.00, 0.25, 0.50 and 0.75) hydride increases from 0.129 wt% for x=0.00 to 0.421 wt% for x=0.75 at 293 K. The pressure–concentration isotherm (P–C–T) curves show that no hydrogen equilibrium pressure plateau has been observed for CeMnAl hydride while the slope of the plateau become flatter and longer with increasing Ni content. Meanwhile, the enthalpy change (ΔH⁰) and the entropy change (ΔS⁰) of the hydrides for dehydrogenization shift from −67.44 kJ mol⁻¹ (x=0.00) to 21.16 kJ mol⁻¹ (x=0.75) and from −0.24 kJ mol⁻¹ K⁻¹ (x=0) to −0.03 kJ mol⁻¹ K⁻¹ (x=0.75), respectively. With increasing Ni content, both ΔH⁰ and ΔS⁰ for dehydrogenization shift to the positive direction and make alloy hydrides more stable and hydrogen desorption much easier.     

Analysis of heat transfer and pressure drop characteristics in an offset strip fin heat exchanger

A steady-state three-dimensional numerical model was used to study the heat transfer and pressure drop characteristics of an offset strip fin heat exchanger. Water was the heat transfer medium, and the Reynolds number Re_dh ranged from 10 to 3500. Variations in the Fanning friction factor f and the Colburn heat transfer factor j relative to Re_dh were observed. General correlations for the f and j factors were derived, and these could be used to analyze fluid flow and heat transfer characteristics of offset strip fins in the laminar, transition, and turbulent regions. Finally, three performance criteria (j/f, j/f^1/3, and JF) were adopted, and the best performance criteria for the cases Pr = 7 and Pr = 50 were chosen to be JF and j/f^1/3, respectively.     

Ionic conduction in Sn1−xScxP2O7 for intermediate temperature fuel cells

A novel series of mixed ion conductors, Sn_1−xSc_xP₂O₇ (x = 0.03, 0.06, 0.09, 0.12), were synthesized by a solid-state reaction method. The conduction behaviors of the ion conductors in wet hydrogen atmosphere were investigated by some electrochemical methods including AC impedance spectroscopy, gas concentration cells in the temperature range of 323-523 K. It was found that the doping limit of Sc³⁺ in SnP₂O₇ was between 9 mol% and 12 mol%. The highest conductivity was observed to be 2.76 × 10⁻² S cm⁻¹ for the sample of x = 0.06 under wet H₂ atmosphere at 473 K. The ionic conduction was contributed mainly to proton and partially to oxide ion in wet hydrogen atmosphere from 373 K to 523 K. The H₂/air fuel cells using Sn_1−xSc_xP₂O₇ (x = 0.03, 0.06, 0.09) as electrolytes (1.7 mm in thickness) generated the maximum power densities of 11.16 mW cm⁻² for x = 0.03, 25.02 mW cm⁻² for x = 0.06 and 14.34 mW cm⁻² for x = 0.09 at 423 K, respectively. The results indicated that Sn_1−xSc_xP₂O₇ is a promising solid electrolyte system for intermediate temperature fuel cells.     

Hydrodynamics and heat transfer in swirl flow under conditions of one-side heating. Part 1: Pressure drop and single-phase heat transfer

The paper gives the basic results of experimental investigation of hydrodynamics and heat transfer in heat-absorbing devices of the ITER thermonuclear reactor, which are subjected to one-side heating. The entire array of experimental data is obtained in the following range of parameters of water flow: pressure p = 0.7–2.0 MPa, mass flux G = 340–25,000 kg/(m² s), inlet water temperature T_in = 15–60 °C. The experiments are performed with turbulent swirl flows of water for twisted tapes with the flow swirl coefficient k = 0.90, 0.66, 0.49, 0.39, 0.25, 0.19, and 0, as well for test sections without a tape. Given in the first part of the paper are the data on pressure drop and single-phase convective heat transfer. Appropriate calculation formulas are derived, which reliably generalize the experimental data.     

Intermediate temperature ionic conduction in Sn1−xGaxP2O7

A novel series of samples Sn_1−xGa_xP₂O₇ (x = 0.00, 0.01, 0.03, 0.06, 0.09, 0.12, 0.15) are synthesized by solid state reaction. XRD patterns indicate that the samples of x = 0.00 − 0.09 exhibit a single cubic phase structure, and the doping limit of Ga³⁺ in Sn_1−xGa_xP₂O₇ is x = 0.09. The protonic and oxide-ionic conduction in Sn_1−xGa_xP₂O₇ are investigated using some electrochemical methods at intermediate temperatures (323-523 K). It is found that the samples exhibit appreciable protonic conduction in hydrogen atmosphere, and a mixed conduction of oxide-ion and electron hole in dry oxygen-containing atmosphere. The highest conductivities are observed for the sample of x = 0.09 to be 4.6 × 10⁻² S cm⁻¹ in wet H₂ and 2.9 × 10⁻² S cm⁻¹ in dry air at 448 K, respectively. The H₂/air fuel cell using x = 0.09 as electrolyte (thickness: 1.45 mm) generates a maximum power density of 19.2 mW cm⁻² at 423 K and 22.1 mW cm⁻² at 448 K, respectively.     

La1−xSrxCoO3 (x = 0.1-0.5) as the cathode catalyst for a direct borohydride fuel cell

Direct borohydride fuel cells (DBFCs), with a series of perovskite-type oxides La_1−xSr_xCoO₃ (x = 0.1-0.5) as the cathode catalysts and a hydrogen storage alloy as the anode catalyst, are studied in this paper. The structures of the perovskite-type catalysts are mainly La_1−xSr_xCoO₃ (_x = 0.1-0.5) oxides phases. However, with the increase of strontium content, the intensities of the X-ray diffraction peaks of the impure phases La₂Sr₂O₅ and SrLaCoO₄ are gradually enhanced. Without using any precious metals or expensive ion exchange membranes, a maximum current density of 275 mA cm⁻² and a power density of 109 mW cm⁻² are obtained with the Sr content of x = 0.2 at 60 °C for this novel type of fuel cell.     

Artificial neural network techniques for the determination of condensation heat transfer characteristics during downward annular flow of R134a inside a vertical smooth tube

In this study, the best artificial intelligence method is investigated to estimate the measured convective heat transfer coefficient and pressure drop of R134a flowing downward inside a vertical smooth copper tube having an inner diameter of 8.1 mm and a length of 500 mm during annular flow numerically. R134a and water are used as working fluids in the tube side and annular side of a double tube heat exchanger, respectively. The ANN training sets have the experimental data of in-tube condensation tests including six different mass fluxes of R134a such as 260, 300, 340, 400, 456 and 515 kg m^− 2 s^− 1, two different saturation temperatures of R134a such as 40 and 50 °C and heat fluxes ranging from 10.16 to 66.61 kW m^− 2. The quality of the refrigerant in the test section is calculated considering the temperature and pressure obtained from the experiment. The pressure drop across the test section is directly measured by a differential pressure transducer. Input of the ANNs are the measured values of test section such as mass flux, heat flux, the temperature difference between the tube wall and saturation temperature, average vapor quality, while the outputs of the ANNs are the experimental condensation heat transfer coefficient and measured pressure drop in the analysis. Condensation heat transfer characteristics of R134a are modeled to decide the best approach using several artificial neural network (ANN) methods such as multilayer perceptron (MLP), radial basis networks (RBFN), generalized regression neural network (GRNN) and adaptive neuro-fuzzy inference system (ANFIS). Elimination process of the ANN methods is performed by means of 183 data points, divided into two sets randomly, obtained in the experiments. Sets of test and training/validation include 33 and 120/30 data points respectively for the elimination process. Validation process, in terms of various experimental conditions, is done by means of 368 experimental data points having 68 data points for test set and 300 data points for training set. In training phase, 5-fold cross validation is used to determine the best value of ANNs control parameters. The ANNs performances were measured by means of relative error criteria with the usage of unknown test sets. The performance of the method of multi layer perceptron (MLP) with 5-13-1 architecture and radial basis function networks (RBFN) were found to be in good agreement, predicting the experimental condensation heat transfer coefficient and pressure drop with their deviations being within the range of ± 5% for all tested conditions. Dependency of outputs of the ANNs from input values is also investigated in the paper.     

SrCo1−xSbxO3−δ perovskite oxides as cathode materials in solid oxide fuel cells

The SrCo_1−xSb_xO_3−δ (x = 0.05, 0.1, 0.15 and 0.2) system was tested as possible cathode for solid oxide fuel cells (SOFCs). X-ray diffraction results show the stabilization of a tetragonal P4/mmm structure with Sb contents between x = 0.05 and x = 0.15. At x = 0.2 a phase transition takes place and the material is defined in the cubic Pm-3m space group. In comparison with the undoped hexagonal SrCoO₃ phase, the obtained compounds present high thermal stability without abrupt changes in the expansion coefficient. In addition, a great enhancement of the electrical conductivity was observed at low and intermediate temperatures (T ≤ 800 °C). The sample with x = 0.05 displays the highest conductivity value that reaches 500 S cm⁻¹ at 400 °C and is over 160 S cm⁻¹ in the usual working conditions of a cathode in SOFC (650-900 °C). Moreover, the impedance spectra of the SrCo_1−xSb_xO_3−δ/Ce_0.8Nd_0.2O_2−δ/SrCo_1−xSb_xO_3−δ (x ≥ 0.05) symmetrical cells reveal polarization resistances below 0.09 Ω cm² at 750 °C which are much smaller than that displayed by the pristine SrCoO_3−δ sample. The composition with x = 0.05 shows the lowest ASR values ranging from 0.009 to 0.23 Ω cm² in the 900-600 °C temperature interval with an activation energy of 0.82 eV.     

Some physical aspects of hydrogen behaviour in the H-Storage bcc alloys Ti35VxCr65−x, Ti40VxMn50−xCr10 and TixCr97.5−xMo2.5

Hydrogen absorption/desorption has been investigated in the three series of solid solution bcc alloys Ti₃₅V_xCr_65−x (x = 18,22), Ti₄₀V_xMn_50−xCr₁₀ (x = 32,36) and Ti_xCr_97.5−xMo_2.5 (x = 43,46). It has been found that the H absorption at pressures smaller than 1 bar can only occur after elimination of the oxide films by heating the alloys to temperatures higher than 600 K. Hydrogen desorption from pre-loaded materials (n_H = H/Me ≤ 0.27) takes place on heating at much lower temperatures in the Ti₄₀V_xMn_50−xCr₁₀ and Ti₃₅V_xCr_65−x than in the Ti_xCr_97.5−xMo_2.5 alloys. The H diffusion parameters W and D_o deduced from high temperature (>450 K) absorption experiments are as follows: W = 0.318 ± 0.005 eV, D_o = (4 ± 1)×10⁻⁷ m²/s for Ti₄₀V_xMn_50−xCr₁₀; W = 0.32 ± 0.02 eV, D_o = (3 ± 2)×10⁻⁷ m²/s for Ti₃₅V_xCr_65−x; W = 0.79 ± 0.06 eV, D_o = (4 ± 2)×10⁻⁸ m²/s for Ti_xCr_97.5−xMo_2.5. The higher value of the activation energy for H diffusion in Mo containing alloys is most likely due to remarkable attractive interactions between H and Mo atoms.