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.     

High-pressure Condensing Refrigerant Flows Through Microchannels,Part II: Heat Transfer 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. Part I presented pressure drop measurements and a two-phase pressure drop model. In this paper, Part II of the study, a condensation heat transfer model is presented. Heat transfer coefficients were measured during condensation of R404A in circular channels (inner diameter = 0.86, 1.55, 3.05 mm) over the entire quality range. The saturation temperature was varied from 30 to 60°C, and mass flux from 200 to 800 kg m^-2 s^-1, to evaluate their effects on condensation heat transfer coefficient. The heat transfer model is developed using a microchannel flow regime map and the void fraction model previously developed by the authors. The resulting model predicts 93.6% of the data within ±25%. The model exhibited good agreement with data from condensing ammonia and carbon dioxide, predicting 84.8% and 97% of their data within ±25%, respectively.     

Numerical Investigation of Jet Impingement Cooling of a Flat Plate with Carbon Dioxide at Supercritical Pressures

Confined round jet impingement cooling of a flat plate at constant heat flux with carbon dioxide at supercritical pressures was investigated numerically. The pressure ranged from 7.8 to 10.0 MPa, which is greater than the critical pressure of carbon dioxide, 7.38 MPa. The inlet temperature varied from 270 to 320 K and the heat flux ranged from 0.6 to 1.6 MW/m². The shear-stress transport turbulence model was used and the numerical model was validated by comparison with experimental results for jet impingement heating with hot water at supercritical pressures. Radial conduction in the jet impingement plate was also considered. The sharp variations of the thermal-physical properties of the fluid near the pseudocritical point significantly influence heat transfer on the target wall. For a given heat flux, the high specific heat near the wall for the proper inlet temperature and pressure maximizes the average heat transfer coefficient. For a given inlet temperature, the heat transfer coefficient remains almost unchanged with increasing surface heat flux at first and then decreases rapidly as the heat flux becomes higher due to the combined effects of the thinner high specific heat layer and the smaller thermal conductivity at higher temperature.     

LAMINAR FORCED CONVECTION CONDENSATION OF SATURATED VAPORS IN THE NEAR-CRITICAL REGION

Abstract

A numerical analysis is presented for laminar forced convection condensation of saturated vapors of water and carbon dioxide on a flat surface in the reduced temperature range T_s / T_c = 0.990-0.999. The heat transfer coefficient in the region T_s / T_c < 0.998 can be correlated by using Fujii and Uehara's equation when the representative physical properties art evaluated at the film temperature. The reduction of the condensation mass flux or the heat flux at the vapor-liquid interface due to the convection term in the condensate film is expressed as a function of the phase change number with the average isobaric specific heat.     

Dropwise Condensation Heat Transfer on Plasma-Ion-Implanted Small Horizontal Tube Bundles

Stable dropwise condensation of saturated steam was achieved on stainless-steel tube bundles implanted with nitrogen ions by plasma ion implantation. For the investigation of the condensation heat transfer enhancement by plasma ion implantation, a condenser was constructed in order to measure the heat flow and the overall heat transfer coefficient for the condensation of steam on the outside surface of tube bundles. For a horizontal tube bundle of nine tubes implanted with a nitrogen ion dose of 10¹⁶ cm^{? 2}, the enhancement ratio, which represents the ratio of the overall heat transfer coefficient of the implanted tube bundle to that of the unimplanted one, was found to be 1.12 for a cooling-water Reynolds number of about 21,000. The heat flow and the overall heat transfer coefficient were increased by increasing the steam pressure. The maximum overall heat transfer coefficient of 2.22 kW · m^?2· K^?1 was measured at a steam pressure of 2 bar and a cooling-water Reynolds number of about 2,000. At these conditions, more dropwise condensation was formed on the upper tube rows, while the lowest row received more condensate, which converted the condensation form to filmwise condensation.     

Condensation heat transfer coefficients of the zeotropic refrigerant mixture R-22/R-142b in smooth horizonal tubes

Heat transfer coefficients during condensation of the zeotropic refrigerant mixture R-22 with R-142b are presented. Measurements were obtained at different mass fractions in a smooth horizontal tube. All measurements were conducted at a high condensing saturation pressure of 2.43 MPa, which corresponds to a condensation temperature of 60 °C for R-22. The measurements were taken in 8.11 mm inner diameter smooth tubes with lengths of 1 603 mm. The heat transfer coefficients were determined with the Log Mean Temperature Difference equations. It was found that at low mass fluxes, between 40 kg·m⁻²·s⁻¹ to 350 kg·m⁻²·s⁻¹, the refrigerant mass fraction influences the heat transfer coefficient by up to a factor of two. The heat transfer coefficients decrease as the fraction of R-142b is increased. At high mass fluxes, of 350 kg·m⁻²·s⁻¹ and more the heat transfer coefficients were not strongly influenced by the refrigerant mass fraction. The average heat transfer coefficient decreased by only 7% as the refrigerant mass fraction changed from 100% R-22 to 50%/50% R-22/R142b.     

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.     

Evaporation Heat Transfer Coefficients of R-446A

ABSTRACT

The present study investigated the evaporation heat transfer coefficients of R-446A, as a low global warming potential alternative refrigerant to R-410A. The evaporation heat transfer coefficients were obtained by measuring the wall temperature of a straight stainless tube and refrigerant pressure. The heat transfer coefficients were measured for the quality range from 0.05 to 0.95, the mass flux from 100 to 400 kg/m²s, heat flux from 10 to 30 kW/m², and saturation temperature from 5 to 10°C. The evaporation heat transfer coefficient of R-410A was verified by comparing the measured evaporation heat transfer coefficient with the value predicted by the existing correlation. The evaporation heat transfer coefficient of R-446A was measured using a proven experimental apparatus. When the heat flux was 10 kW/m², the evaporation heat transfer coefficient of R-446A was always higher than that of R-410A. But, when the heat flux was 30 kW/m², the evaporation heat transfer coefficient of R-446A was measured to be lower than that of R-410A near the dry-out point. The effect of the tube diameter on the R-446A evaporation heat transfer coefficient was negligible. The effect of saturation pressure on the evaporation heat transfer coefficient was prominent in the low quality region where the nucleate boiling was dominant.     

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.     

Evaporative Cooling Heat Transfer of Water From Hierarchically Porous Aluminum Coating

A study of evaporative cooling of water was conducted using dual-scale hierarchically porous aluminum coating. The coating was created by brazing aluminum powders to a flat aluminum plate. The effects of particle size and thickness on evaporative heat transfer were investigated using average aluminum particle diameters of 27, 70, and 114 µm and average coating thicknesses of 560, 720, and 1200 µm. Constant ambient temperature of 24°C and relative humidity of 50% were provided throughout the study. Evaporative cooling tests on the coated surfaces were compared to the plain surface. Tested dual-scale porous coatings enhanced evaporative heat transfer significantly, compared to that of the plain surface, due to the effective wicking of water to the entire heated area. With particle size increase, both the wickability and dryout heat flux were significantly increased. The dryout heat flux with the particle size of 114 µm was 3.2 times higher than that with the particle size of 27 µm. At the fixed particle size of 70 µm the dryout heat flux increased as thickness increased, which resulted in the maximum dryout heat flux of 10.6 kW/m² and the maximum heat transfer coefficient of 251 W/m²K at the coating thickness of 1200 µm.     

Dropwise condensation heat transfer on ion implanted aluminum surfaces

Stable dropwise condensation (DWC) of saturated steam has been achieved on an aluminum alloy Al 6951 disc with an average surface finish of about 0.15 μm by means of ion beam implantation technology with an ion dose of 10¹⁶ N⁺ cm^?2 and an implantation energy of 20 keV. Measurements of the condensation heat transfer coefficient at steam pressures of 1200 and 1400 mbar were carried out as a function of surface subcooling on vertical plates of the same material which is commonly used for heat transfer equipment. Probably due to alloy inhomogeneities, only on about 50% of the plate surface DWC could be achieved, resulting in a maximum enhancement factor of 2.0 for DWC in comparison with theoretical values calculated by a corrected form of the Nusselt film theory. The heat transfer coefficient increases with increasing steam pressure and decreases with increasing surface subcooling. Furthermore, it was shown that condensation heat transfer cannot be enhanced if the ion implantation does not induce DWC. For the investigations, two different condensers have been used, one for the stability tests on discs and one for the heat transfer measurements on plates.     

Heat Transfer Performance of an Edge-Shaped Finned Tube

The third-generation heat transfer technologies, such as three-dimensional fin and dimple, are still important means of improving energy efficiency and will continue to be challenging issues. This paper presents condensation heat transfer performance of an edge-shaped finned tube fabricated by a ploughing–extruding process. The edge-shaped finned tube integrates more than one heat transfer enhancement technology and can enhance the heat transfer capacity greatly. It is seen that the overall heat transfer coefficient and heat flux increase with inlet velocity of cold water increasing, and decrease with inlet temperature of cold water increasing, whereas the shell-side heat transfer coefficient decreases with inlet velocity of cold water increasing and increases with inlet temperature of cold water increasing. At the same inlet velocity, the shell-side heat transfer coefficient for the edge-shaped finned tube is improved by 5–7 times compared to that of a smooth tube. At the same temperature difference between wall and vapor, the shell-side heat transfer coefficient is also higher than what had been reported in the literature. The shell-side heat transfer coefficient of the edge-shaped finned tube decreases with the increase of fabrication parameter feed at the same inlet velocity or inlet temperature of cold water.     

Asymmetrical Non-Uniform Heat Flux Distributions For Laminar Flow Heat Transfer With Mixed Convection In a Horizontal Circular Tube

Non-symmetric heat flux distributions in terms of gravity in solar collector tubes influence buoyancy-driven secondary flow which has an impact on the associated heat transfer and pressure drop performance. In this study the influence of the asymmetry angle (0°, 20°, 30° and 40°) with regard to gravity for non-uniform heat flux boundaries in a horizontal circular tube was investigated numerically. A stainless steel tube with an inner diameter of 62.68 mm, a wall thickness of 5.16 mm, and a length of 10 m was considered for water inlet temperatures ranging from 290 K to 360 K and inlet Reynolds numbers ranging from 130 to 2000. Conjugate heat transfer was modelled for different sinusoidal type outer surface heat flux distributions with a base-level incident heat flux intensity of 7.1 kW/m². It was found that average internal heat transfer coefficients increased with the circumferential span of the heat flux distribution. Average internal and axial local heat transfer coefficients and overall friction factors were at their highest for symmetrical heat flux cases (gravity at 0º) and lower for asymmetric cases. The internal heat transfer coefficients also increased with the inlet fluid temperature and decreased with an increase in the external heat loss transfer coefficient. Friction factors decreased with an increase in fluid inlet temperature or an increase in the external heat loss transfer coefficients of the tube model.     

Wake Flow and Heat Transfer Due to a Spherical Viscous Droplet

A numerical study on the buoyancy-assisted flow and heat transfer from a liquid spherical droplet falling in fluid medium is made. The investigation is based on the solution of the Navier-Stokes equations together with the energy equation inside and outside the droplet, along with a suitable interface condition. The governing equations for three-dimensional flow and heat transfer are solved through the pressure correction based iterative algorithm, SIMPLE. The Reynolds number for the exterior flow is considered below 300 with the Richardson number in the range 0 ≤ Ri ≤ 1.5. The form of the wake due to the viscous droplet and its influence on heat transfer and drag coefficient are analyzed for a wide range of physical parameters. It is found that by increasing the Reynolds number, the predicted rate of heat transfer is significantly increased for a liquid droplet compared to a solid sphere. The increment of viscosity of the droplet increases the drag experienced by the droplet but reduces the rate of heat transfer. An increase in Richardson number produces an increment in drag coefficient as well as in heat transfer. In order to establish a simplified model for heat transfer due to a viscous droplet, we compared our computed solutions with several empirical correlations for conjugate heat transfer and proposed a model (in absence of buoyancy). We have also investigated the validity of several empirical correlations for the drag coefficient.     

Experimental Assessment of Heat Transfer Correlations for a Small-Scale Evaporative Condenser

This work presents an experimental analysis of a small-scale evaporative condenser, performed on a calorimetric test facility, where heat transfer coefficients are measured and compared to some well-known correlations from the literature. The external flow of air and spray water ranged from 2.2 kg/min to 8.7 kg/min and 4.8 kg/min to 15.0 kg/min, respectively, keeping an average air to water ratio of approximately 1:2, followed by condensation temperatures ranging from 26 to 36°C. The flow pattern map is first determined, followed by the identification of the transition regions based on the void fraction concept. The overall heat transfer coefficient for the condensation zone calculated after the experimental data acquired in the present research was compared to six literature correlations, and the one developed by Tovaras, Bykov, and Gogolin in 1984 provided the better agreement. Local and mean values of the refrigerant heat transfer coefficients did not vary significantly for both single-phase superheated vapor and subcooling liquid. Results are still particular to the evaporative condenser assessed in the present work, and full-scale analysis must be performed in order to build more general correlations.     

Study of viscosity and thermal conductivity effects on condensation characteristics of some new alternative refrigerant mixtures

In this paper, a numerical study of the impact of the transport properties on the condensation characteristics of certain refrigerant mixtures is presented. New correlations have been developed to calculate the thermal conductivity and viscosity of some alternative refrigerant mixtures such as R-507, R-404A, R-407C, and R-410A. In addition, new improved condensation correlations have been developed and presented for predicting the heat transfer coefficient and pressure drop. The results clearly showed that the condensation characteristics were well predicted using the newly proposed correlations with mean deviation of ±10 and 20% for the heat transfer coefficient and pressure drop respectively. Copyright © 2002 John Wiley & Sons, Ltd.     

Filtration Gas Combustion in a Porous Ceramic Annular Burner for Thermoelectric Power Conversion

A numerical study of the combustion of lean methane/air mixtures in a porous media burner is performed using novelty geometry, cylindrical annular space. The combustion process takes place in the porous space located between two pipes, which are filled with alumina beads of 5.6 mm diameter forming a porosity of 0.4. The outer tube diameter of 3.82 cm is isolated; meanwhile the inner tube of 2 cm in diameter is covered by a continuous set of thermoelectric elements (TE) for transforming heat energy into electricity. To achieve and maintain the proper temperature gradient on TE, convective heat losses are considered from the TE. Computer simulations focus on the two-dimensional (2D) temperature analysis and displacement dynamics of the combustion front inside the reactor, depending on the values of the filtration velocity (0.1 to 1.0 m/s), the heat loss coefficient from the internal cylinder (400–1500 W/m²/K), and the fuel equivalence ratio (0.06– 0.5). The conditions that maximized the overall performance of the process of energy conversion are: 0.7 m/s of the filtration velocity, 0.363 of the fuel equivalence ratio and 1500 W/(m^2·K) of the heat transfer coefficient from the internal cylinder, to obtain 2.05 V electrical potential, 21 W of electrical power, and 5.64% of the overall process efficiency. The study shows that the cylindrical annular geometry can be used for converting the energy of combustion from lean gas mixtures into electricity, with a performance similar to the specified by manufacturers of thermoelectric elements (TE).     

Experimental and Numerical Analysis of Heat Transfer in a Tall Vertical Concentric Annular Thermo-siphon at Constant Heat Flux Condition

The present work deals with the numerical and experimental analyses to study the detailed behavior of the thermally induced flow of water in an open vertical annulus, circulating through a cold leg forming a closed loop thermo-siphon. Spatio-temporal behavior of fluid flow is also studied for variety of heat fluxes. The annuli in the present study have a radius ratio of 1.184 and aspect ratio (length to annular gap) equal to 352. The objective of the present work is to quantify the effect of heating on design parameters such as liquid and wall temperatures, mass flow rate, and heat transfer coefficient. Experiments have also been conducted on a similar system with water at constant heat flux of 1 kW/m², 2.5 kW/m², 5 kW/m², 7.5 kW/m², 10 kW/m², 12.5 kW/m² and 15 kW/m². For numerical purpose, a two-dimentional solver has been developed for direct numerical simulation of the essential thermally induced flow dynamics The numerical solution was thus performed for Rayleigh numbers ranging between, 4.4 × 10³ and 6.61 × 10⁴ which correspond to the given heat flux, respectively.     

Transient and steady-state natural convection heat transfer from a heated horizontal concrete cylinder

In refrigeration systems, it is possible to reduce energy consumption (compressor power) and increase COP by decreasing the condensation temperature. Decreasing the condensation temperature can be achieved either by increasing the overall heat transfer coefficient or heat transfer surface area of the condenser. Usually, the radiuses of condenser tubes of domestic refrigerators are quite smaller than the critical radius. Thus, the radius can be increased up to the critical radius by coating the bare condenser tube to increase heat transfer. On the other hand, refrigerators operate discontinuously depending on the ambient temperatures. Coating material stores some of the heat during the working period and continues heat transfer during the off period so that the condenser continues transferring heat while the compressor is not working. Storage effect depends on the specific heat and density of the coating material. Transient and steady-state natural convection heat transfer from a heated horizontal cylinder covered with concrete layer by molding is studied experimentally and numerically to determine the effects of the parameters considered above. The copper and the concrete test cylinders used in the experimental study have a length of 1 m and outer diameter of 9.45 mm and 68.5 mm respectively. The ambient and copper cylinder surface temperatures varied between 20 °C÷30 °C and 30 °C÷50 °C respectively. Constant heat flux was applied to bare and concrete cylinders. Transient heat transfer experiments were performed when bare, and concrete cylinders were reached to steady state condition. Heat transfer rates under transient conditions from bare and concrete horizontal cylinders were compared and heat transfer enhancement was determined. Based on the experimental data average Nusselt numbers were calculated and compared with the well known correlations. Also temperature distributions obtained from numerical simulations were very close to the experimental data. The effect of the decrease in the temperature of the inner copper cylinder surface (condensation temperature) on COP was investigated considering an ideal Carnot refrigeration cycle. It is found that the enhancement in COP of a Carnot refrigeration cycle is 35.7% under transient condition.     

Pool boiling heat transfer of water and nanofluid outside the surface with higher roughness and different wettability

In order to investigate the effect of surface wettability on the pool boiling heat transfer, nucleate pool boiling experiments were conducted with deionized water and silica based nanofluid. A higher surface roughness value in the range of 3.9 ~ 6.0μm was tested. The contact angle was from 4.7° to 153°, and heat flux was from 30kW/m² to 300kW/m². Experimental results showed that hydrophilicity diminish the boiling heat transfer of silica nanofluid on the surfaces with higher roughness. As the increment of nanofluid mass concentration from 0.025% to 0.1%, a further reduction of heat transfer coefficient was observed. For the super hydrophobic surface with higher roughness (contact angle 153.0°), boiling heat transfer was enhanced at heat flux less than 93 kW/m², and then the heat transfer degraded at higher heat flux.