Numerical investigation of nanofluid heat transfer in helically coiled tubes using the four-equation model

Helical coils and nanofluids are among efficient methods for heat transfer augmentation. The present study numerically investigates convective heat transfer with nanofluids in helically coiled tubes. Two boundary conditions are applied to the coil walls; constant temperature and constant heat flux. Heat transfer in nanofluids are mainly investigated using either the homogeneous model or the two-phase model. However, in the present numerical solution, the four-equation model is applied, using slip mechanisms for the base fluid and nanoparticles. Considering that the proposed model is simplified compared to the two-phase model, it can be regarded as an efficient model for numerical solution of heat transfer in nanofluids. Governing equations are solved in the non-dimensional form using the projection algorithm of finite difference method. Water/CuO with a 0.2% volume fraction and water/Ag with a 0.03% volume fraction are examined for validation of numerical results in case of constant temperature and constant heat flux boundary conditions, respectively. The obtained results show a better agreement of this model with respect to experimental data, compared to the homogeneous model.     

Experimental investigation on heat transfer characteristics of supercritical CO2 cooled in horizontal helically coiled tube

An experimental investigation on the heat transfer characteristics of supercritical CO₂ during gas cooling process in a helically coiled tube is conducted. The experimental data are obtained over a mass flux range of 79.6–238.7 kg m⁻² s⁻¹, an inlet pressure range of 7.5–9.0 MPa and a mean bulk temperature of 23.0–53.0 °C. The effects of mass flux, bulk temperature and pressure on the heat transfer coefficient for helically coiled tubes are investigated. A comparative analysis of the gravitational buoyancy and the heat transfer coefficient is carried out between helically coiled tubes and straight tubes. A new heat transfer correlation of the supercritical CO₂ in the horizontal helically coiled tube is proposed based on the experimental data. The maximum error between the predicted results of the new correlation and the experimental data is 20%.     

Experimental study of condensation heat transfer of R-404A in helically coiled tubes

In this research, the heat transfer coefficients of R-404A vapor condensation inside helically coiled tubes are studied, experimentally. The effects of different coil pitches and curvature radii at different vapor qualities and mass velocities are considered. The vapor is condensed inside the helically coiled tubes by transferring heat to the cooling water flowing in annulus. Results show that the coil diameter has significant effect on condensation heat transfer coefficient. By decreasing the coil diameter or increasing the Dean number, the heat transfer coefficient is increased as the highest value is obtained at curvature radius of 4.35 cm which is 45% greater than the corresponding figure of curvature radius of 7.65 cm at mass velocity of 125 kg m⁻² s⁻¹. Also, at low vapor qualities, the coil pitch effect is more pronounced. Finally, based on the results, a new correlation is developed to evaluate the condensation heat transfer coefficient of R-404A inside helically coiled tubes.     

Experimental investigation on flow condensation heat transfer and pressure drop of R170 in a horizontal tube

Condensation heat transfer and pressure drop of R170 were studied experimentally in a horizontal tube with inner diameter of 4 mm. The tests were conducted at saturation pressures from 1 MPa to 2.5 MPa, mass fluxes from 100 kg (m²∙s)⁻¹ to 250 kg (m²∙s)⁻¹ and average heat fluxes from 55.3 kW m⁻² to 96.3 kW m⁻² over the entire vapor quality range. The effects of vapor quality, mass flux and saturation pressure on condensation heat transfer and pressure drop were examined and analyzed. The experimental data were compared with various well-known correlations of condensation heat transfer coefficient and pressure drop. The comparison results showed that Koyama et al. correlation agreed with the experimental heat transfer coefficient with a mean absolute relative deviation less than 25%, and the Yan and Lin correlation can accurately predict the experimental pressure drop with a mean absolute relative deviation less than 18%.     

Experimental studies on pressure drop characteristics of cryogenic cross-counter flow coiled finned tube heat exchangers

Cross-counter flow coiled finned tube heat exchangers used in medium capacity helium liquefiers/refrigerators were developed in our lab. These heat exchangers were developed using integrated low finned tubes. Experimental studies have been performed to know the pressure drop characteristics of tube side and shell side flow of these heat exchangers. All experiments were performed at room temperature in the Reynolds number range of 3000-30,000 for tube side and 25-155 for shell side. The results of present experiments indicate that available correlations for tube side can not be used for prediction of tube side pressure drop data due to complex surface formation at inner side of tube during formation of fins over the outer surface. Results also indicate that surface roughness effect becomes more pronounced as the value of di/D_m increases. New correlations based on present experimental data are proposed for predicting the friction factors for tube side and shell side.     

Experimental investigation of heat transfer by natural convection in an enclosure

An experimental study has been made of heat transfer by natural convection from a thin wire in a cylindrical enclosure filled with liquid. The dependence of the convection coefficient on the parameter GrPr has been found with different lengths of measuring interval and for various inclinations of the measuring tube to the horizontal.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 15, No. 6, pp. 1124–1127, December, 1968.     

Experimental investigation of evaporation heat transfer coefficient and pressure drop of R-410A in a multiport mini-channel

The evaporation heat transfer coefficient and pressure drop of R-410A flowing through a horizontal aluminium rectangular multiport mini-channel having 3.48 mm hydraulic diameter are experimentally investigated. The test runs are performed at mass flux ranging between 200 and 400 kg/m² s. The heat fluxes are between 5 and 14.25 kW/m² and the saturation temperatures range between 10 and 30 °C. The pressure drop across the test section is directly measured by a differential pressure transducer. The effects of the imposed wall heat flux, mass flux, vapour quality, and saturation temperature on the evaporation heat transfer and pressure drop are also discussed. The results from the present experiment are compared with those obtained from the existing correlation. New correlations for the evaporation heat transfer coefficient and pressure drop of R-410A flowing through a multiport mini-channel are proposed for practical applications.     

An experimental investigation of saturated flow boiling heat transfer and pressure drop in square microchannels

In this study, saturated flow boiling characteristics of deionized water in parallel microchannels are investigated experimentally. The silicone microchannel heat sink consists of 29 parallel square microchannels having hydraulic diameters of 150 µm. Experiments have been conducted for four different values of the mass flux consisting of 51, 64.5, 78 and 92.6 kg/m²s and heat flux values from 59.3 to 84.1 kW/m². Inlet temperature of deionized water is kept at 50 ± 1 °C. Heat transfer and pressure drop are examined for varying values of the governing parameters. Simultaneous high-speed video images have been taken as well as temperature and pressure measurements. The flow visualization results lead to key findings for flow boiling instabilities and underlying physical mechanisms of heat transfer in microchannels. Quasi-periodical rewetting and drying, rapid bubble growth and elongation toward both upstream and downstream of the channels and reverse flow are observed in parallel microchannels.     

电冰箱用微通道冷凝器制冷剂侧传热与压降特性试验研究

建立电冰箱换热器试验台,对具有百叶窗翅片的微通道冷凝器制冷剂侧的传热和压降进行测试。结果表明:随着制冷剂质量流速的增加,冷凝器换热量、换热系数及制冷剂流动压降均增大,在冷凝压力为1.46MPa,制冷剂质量流速从90 kg/(m~2·s)增加到150 kg/(m~2·s)时,换热量、换热系数和压降分别增加63%,116%和166%;随着冷凝压力的升高,换热量增大,换热系数减小,在制冷剂质量流速为150 kg/(m~2·s)时,冷凝压力为1.46 MPa与冷凝压力为1.16 MPa相比,换热量增加12%,换热系数降低39%。     

Numerical investigation of heat transfer enhancement in a square ventilated cavity with discrete heat sources using nanofluid

A numerical study of a laminar mixed convection problem in a ventilated square cavity partially heated from bellow is carried out. The fluid in the cavity is a water-based nanofluid containing Cu nanoparticles. The effects of monitoring parameters, namely, Richardson number, Reynolds number, and solid volume fraction on the streamline and isotherm contours as well as average Nusselt number along the two heat sources are analyzed. The computation is performed for Richardson number ranging from 0.1 to 10, Reynolds number from 10 to 500, and the solid volume fraction from 0 to 0.1. The results show that by adding nanoparticles to the base fluid and increasing both Reynolds and Richardson numbers the heat transfer rate is enhanced. It is also found, regardless of the Richardson and Reynolds numbers, and the volume fraction of nanoparticles, the highest heat transfer enhancement occurs at the left heat source surface.     

Condensation heat transfer and pressure drop characteristics of CO2 in a microchannel

The condensation heat transfer coefficient and pressure drop of CO₂ in a multiport microchannel with a hydraulic diameter of 1.5 mm was investigated with variation of the mass flux from 400 to 1000 kgm⁻²s⁻¹ and of the condensation temperature from −5 to 5 °C. The heat transfer coefficient and pressure drop increased with the decrease of condensation temperature and the increase of mass flux. However, the rate of increase of the heat transfer coefficient was retarded by these changes. The gradient of the pressure drop with respect to vapor quality is significant with the increase of mass flux. The existing models for heat transfer coefficient overpredicted the experimental data, and the deviation increased at high vapor quality and at high heat transfer coefficient. The smallest mean deviation of ±51.8% was found by the Thome et al. model. For the pressure drop, the Mishima and Hibiki model showed mean deviation of 29.1%.     

Experimental investigation of heat transfer in the evaporator of a water heat pipe

The applicability of Gilmour's correlation equation in describing the heat transfer in the evaporator of a low-temperature heat pipe was experimentally confirmed.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 33, No. 2, pp. 250–254, August, 1977.     

Performance of heat transfer and pressure drop in superconducting cable former

This study was performed of the heat transfer and the pressure drop in superconducting cable former of more realistic geometry. Liquid nitrogen passes through the former for cooling. Corrugated pipes are used as formers to ensure flexibility, and the ratio of pitch and depth of corrugation p/e is less than 5, which is out of the range of normal usage. Range of the test was p/e=1–15, e/D=0.039–0.118. The heat transfer and the pressure drop were proportional to the corrugation depth, which is the same with the results reported by previous researchers. But the effect of pitch was different from the results of the range p/e>10. The heat transfer and the pressure drop had the maximum values on the range of p/e=5–10. Based on the numerical results, discussion was made on the design of superconducting cable former.     

Effects of curvature ratio on forced convection and entropy generation of nanofluid in helical coil using two-phase approach

Applying nanofluid and helical coils are two effective methods for thermal performance enhancement. Combination of these techniques could improve the energy efficiency of thermal equipment dramatically. In this study, a numerical analysis of nanofluid flowing in helical coil with constant wall temperature boundary condition was performed to evaluate nanofluid superiority over the base fluid. Forced convective heat transfer and entropy generation of aqueous Al₂O₃ nanofluid with temperature dependent properties were investigated. Eulerian two-phase mixture model was employed for nanofluid modeling and governing mass, momentum, energy, and volume fraction equations were solved using finite volume method. Simulations covered a range of nanoparticle volume fraction of 1–3%, Reynolds number from 200 to 2000, and curvature ratio of 0.05–0.2. In order to evaluate the heat transfer performance, a parameter referred as thermo-hydrodynamic performance index was applied. Also, entropy generation analysis was performed to examine the efficiency of the helical coil and nanofluid. The results demonstrate that performance index enhances by decreasing the Reynolds number and the increasing nanoparticle concentration. The best thermo-hydrodynamic performance can be obtained at low Reynolds number, high nanoparticle volume fraction, and large curvature ratio. Increasing curvature ratio decreases the ratio of local entropy generation by nanofluid to the base fluid. So, utilization of water based Al₂O₃ nanofluid in higher curvature ratio is more efficient from irreversibility point of view.     

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.     

Experimental and numerical study of heat transfer characteristics using the heat balance in a linear compressor

A linear compressor was divided into several control volumes, and its thermal performance was analyzed by maintaining the whole energy balance throughout the heat transfer analysis for each control volume. During the steady state performance test of the compressor, basic measurements of the temperature and pressure were performed by using a calorimeter. First, the energy flow of each control volume was defined using the results of the measurements, and then heat transfer analysis was conducted. Next, the thermal network was defined by using a correlation for the energy balance. A software tool for compressor simulation was developed using a thermal network analysis in order to estimate the energy efficiency ratio (EER) according to the changes in thermal properties of the compressor. Experimental results show that the simulation tool is satisfied up to a 5% change in the EER according to variations in the ambient temperature and mixing ratio of the refrigerant.     

Experimental investigation of heat and mass transfer during condensation of water vapor from humid air on a vertical surface under natural convection conditions

Results are presented of an experimental investigation of the heat and mass transfer accompanying condensation of water vapor from vapor-air mixtures with low volumetric vapor contents (_o.v 2.1%). Empirical formulas are proposed for the heat transfer coefficient and the rate of condensation.