Laminar convective heat transfer and pressure drop in the corrugated channels

The results of the heat transfer characteristics and pressure drop in the corrugated channel under constant heat flux are presented in the present study. The test section is the channel with two opposite corrugated plates which all configuration peaks lie in an in-line arrangement. The corrugated plates with three different corrugated tile angles of 20°, 40°, and 60° are tested with the height of the channel of 12.5 mm. The experiments are done for the heat flux and the Reynolds number in the ranges of 0.5–1.2 kW/m² and 500–1400, respectively. Effect of relevant parameters on the heat transfer characteristics and pressure drop are discussed. Due to the presence of recirculation zones, the corrugated surface has significant effect on the enhancement of heat transfer and pressure drop.     

Experimental study on condensation heat transfer enhancement and pressure drop penalty factors in four microfin tubes

Heat transfer and pressure drop characteristics of four microfin tubes were experimentally investigated for condensation of refrigerants R134a, R22, and R410A in four different test sections. The microfin tubes examined during this study consisted of 8.92, 6.46, 5.1, and 4 mm maximum inside diameter. The effect of mass flux, vapor quality, and refrigerants on condensation was investigated in terms of the heat transfer enhancement factor and the pressure drop penalty factor. The pressure drop penalty factor and the heat transfer enhancement factor showed a similar tendency for each tube at given vapor quality and mass flux. Based on the experimental data and the heat-momentum analogy, correlations for the condensation heat transfer coefficients in an annular flow regime and the frictional pressure drops are proposed.     

Thermal performance and pressure drop of the helical-coil heat exchangers with and without helically crimped fins

In the present study, the thermal performance and pressure drop of the helical-coil heat exchanger with and without helical crimped fins are studied. The heat exchanger consists of a shell and helically coiled tube unit with two different coil diameters. Each coil is fabricated by bending a 9.50 mm diameter straight copper tube into a helical-coil tube of thirteen turns. Cold and hot water are used as working fluids in shell side and tube side, respectively. The experiments are done at the cold and hot water mass flow rates ranging between 0.10 and 0.22 kg/s, and between 0.02 and 0.12 kg/s, respectively. The inlet temperatures of cold and hot water are between 15 and 25 °C, and between 35 and 45 °C, respectively. The cold water entering the heat exchanger at the outer channel flows across the helical tube and flows out at the inner channel. The hot water enters the heat exchanger at the inner helical-coil tube and flows along the helical tube. The effects of the inlet conditions of both working fluids flowing through the test section on the heat transfer characteristics are discussed.     

Prediction of heat transfer and flow characteristics in helically coiled tubes using artificial neural networks

In this study, Artificial Neural Network (ANN) models were developed to predict the heat transfer and friction factor in helically coiled tubes. The experiments were carried out with hot fluid in coiled tubes which placed in a cold bath. Coiled tubes with various curvature ratios and coil pitches (nine Layouts) were used. The output data of the ANNs were Nusselt number and friction factor. The validity of the method was evaluated through a test data set, which were not employed in the training stage of the network. Moreover, the performance of the ANN model for estimating the Nusselt number and friction factor in the coiled tubes was compared with the existing empirical correlations. The results of this comparison show that the ANN models have a superior performance in predicting Nusselt number and friction factor in the coiled tubes.     

Performance evaluation of flattened tube in boiling heat transfer enhancement and its effect on pressure drop

An experimental investigation has been carried out to study heat transfer and pressure drop characteristics of R-134a flow boiling inside a horizontal plain tube and different flattened tubes. Round copper tubes with an inner diameter of 8.7 mm were flattened into an oblong shape with an internal height of 6.6 mm, 5.5 mm, 3.8 mm, and 2.8 mm. The test apparatus was basically a vapor compression refrigeration system equipped with all necessary measuring instruments. Analysis of the collected data showed that, by flattening the round tube, the heat transfer coefficient and pressure drop increased simultaneously. The performance of these tubes from the point of view of heat transfer enhancement and pressure drop increasing were evaluated. It was concluded that, the tube with an internal height of 5.5 mm has the best performance compared with the other flattened tubes. Finally, based on the present experimental pressure drop data, a correlation was developed to estimate the pressure drop in flattened tubes. This correlation predicts the experimental data of the present study within an error band of ± 20%.     

Experimental investigation of twisted tape inserts performance on condensation heat transfer enhancement and pressure drop

A comprehensive experimental investigation is conducted on the augmentation of heat transfer coefficients and pressure drop during condensation of HFC-134a in a horizontal tube at the presence of different twisted tape inserts. The test section is a 1.04 m long double-tube counter-flow heat exchanger. The refrigerant flows in the inner copper and the cooling water flows in annulus. The experiments are performed for a plain tube and four tubes with twisted tapes inserts of 6, 9, 12 and 15 twist ratios. The pressure drop is directly measured by a differential pressure transducer. It is found that the twisted tape with twist ratio of 6 gives the highest enhancement in the heat transfer coefficient and the maximum pressure drop compared to the plain tube on a nominal area basis. For this case the enhancement in heat transfer and the pressure drop are increased by 40 and 240% in comparison with to the plain tube. It is observed that the twisted tape with the twist ratio of 9 has the best performance enhancing the heat transfer with the minimum pressure drop. Also empirical correlations are developed to predict smooth tube and swirl flow pressure drop. Predicted results are compared to experimental data and it is found that these correlations are reliable for pressure drop estimation.     

Evaporating heat transfer and pressure drop of hydrocarbon refrigerants in 9.52 and 12.70 mm smooth tube

Experimental results of heat transfer characteristic and pressure gradients of hydrocarbon refrigerants R-290, R-600a, R-1270 and HCFC refrigerant R-22 during evaporating inside horizontal double pipe heat exchangers are presented. The test sections have one tube diameter of 12.70 mm with 0.86 mm wall thickness, another tube diameter of 9.52 mm with 0.76 mm wall thickness was used for this study. The local evaporating heat transfer coefficients of hydrocarbon refrigerants were higher than those of R-22. The average evaporating heat transfer coefficient increased as the mass flux increased. It is showed the higher values in hydrocarbon refrigerants than R-22. Comparing the heat transfer coefficient of experimental results with that of other correlations, the obtained results from the experiments had coincided with most of the Kandlikar’s correlation. Hydrocarbon refrigerants have higher pressure drop than R-22 in 12.7 mm and 9.52 mm. This results form the study can be used in the case of designing heat transfer exchangers using hydrocarbons as the refrigerant for the air-conditioning systems.     

Turbulent heat transfer enhancement in a heat exchanger using helically corrugated tube

The augmentation of convective heat transfer in a single-phase turbulent flow by using helically corrugated tubes has been experimentally investigated. Effects of pitch-to-diameter ratio (P/D_H = 0.18, 0.22 and 0.27) and rib-height to diameter ratio (e/D_H = 0.02, 0.04 and 0.06) of helically corrugated tubes on the heat transfer enhancement, isothermal friction and thermal performance factor in a concentric tube heat exchanger are examined. The experiments were conducted over a wide range of turbulent fluid flow of Reynolds number from 5500 to 60,000 by employing water as the test fluid. Experimental results show that the heat transfer and thermal performance of the corrugated tube are considerably increased compared to those of the smooth tube. The mean increase in heat transfer rate is between 123% and 232% at the test range, depending on the rib height/pitch ratios and Reynolds number while the maximum thermal performance is found to be about 2.3 for using the corrugated tube with P/D_H = 0.27 and e/D_H = 0.06 at low Reynolds number. Also, the pressure loss result reveals that the average friction factor of the corrugated tube is in a range between 1.46 and 1.93 times over the smooth tube. In addition, correlations of the Nusselt number, friction factor and thermal performance factor in terms of pitch ratio (P/D_H), rib-height ratio (e/D_H), Reynolds number (Re), and Prandtl number (Pr) for the corrugated tube are determined, based on the curve fitting of the experimental data.     

Pressure drop and thermal characteristics of CuO-base oil nanofluid laminar flow in flattened tubes under constant heat flux

An experimental investigation has been carried out to study the heat transfer and pressure drop characteristics of nanofluid flow inside horizontal flattened tubes under constant heat flux. The nanofluid is prepared by dispersion of CuO nanoparticle in base oil and stabilized by means of an ultrasonic device. Nanofluids with different particle weight concentrations of 0.2%, 0.5%, 1% and 2% are used. Copper tubes of 11.5 mm I.D. are flattened into oblong shapes and used as test sections. The nanofluid flowing inside the tube is heated by an electrical heating coil wrapped around it. Required data are acquired for laminar and hydrodynamically fully developed flow inside round and flattened tubes.The effect of different parameters such as flow Reynolds number, flattened tube internal height and nanofluid particle concentration on heat transfer coefficient and pressure drop of the flow is studied. Observations show that the heat transfer performance is improved as the tube profile is flattened. Flattening the tube profile resulted in pressure drop increasing. In addition, the heat transfer coefficient as well as pressure drop is increased by using nanofluid instead of base fluid. Furthermore, the performance evaluation of the two enhanced heat transfer techniques studied in this investigation shows that applying flattened tubes instead of the round tube is a more effective way to enhance the convective heat transfer coefficient compared to the second method which is using nanofluids instead of the base liquid.     

Numerical investigation on the single phase forced convection heat transfer characteristics of TiO2 nanofluids in a double-tube counter flow heat exchanger

In this study, forced convection flows of nanofluids consisting of water with TiO₂ and Al₂O₃ nanoparticles in a horizontal tube with constant wall temperature are investigated numerically. The horizontal test section is modeled and solved using a CFD program. Palm et al.'s correlations are used to determine the nanofluid properties. A single-phase model having two-dimensional equations is employed with either constant or temperature dependent properties to study the hydrodynamics and thermal behaviors of the nanofluid flow. The numerical investigation is performed for a constant particle size of Al₂O₃ as a case study after the validation of its model by means of the experimental data of Duangthongsuk and Wongwises with TiO₂ nanoparticles. The velocity and temperature vectors are presented in the entrance and fully developed region. The variations of the fluid temperature, local heat transfer coefficient and pressure drop along tube length are shown in the paper. Effects of nanoparticles concentration and Reynolds number on the wall shear stress, Nusselt number, heat transfer coefficient and pressure drop are presented. Numerical results show the heat transfer enhancement due to presence of the nanoparticles in the fluid in accordance with the results of the experimental study used for the validation process of the numerical model.     

Heat transfer and pressure drop characteristics of forced convective evaporation in horizontal tubes with coiled wire inserts

Heat transfer enhancement and pressure drop increasing during evaporation of R-134a due to the presence of coiled wire insert inside a horizontal evaporator was studied experimentally. The test evaporator was an electrically heated copper tube of 1200 mm length and 7.5 mm inside diameter. Helically wire coils with different wire diameters of 0.5, 0.7, 1.0 and 1.5 mm and different coil pitches of 5, 8, 10 and 13 mm were made and used in full length of the test evaporator. For each inserted tube and also the plain tube, several test runs were carried out with different mass velocities and heat fluxes. From analysis of acquired data, it was found that the coiled wire inserts enhance the heat transfer coefficient but with a higher penalty due to the increasing of pressure drop, in comparison to that for the plain flow. An empirical correlation has been developed to predict the heat transfer coefficient during evaporation inside a horizontal tube in the presence of a coiled wire insert.     

Transient convective heat transfer of steam-water two-phase flow in a helical tube under pressure drop type oscillations

Experiments for subcooled water flow and for steam-water two-phase flow were conducted to investigate the effects of pulsation upon transient heat transfer characteristics in a closed-circulation helical-coiled tube steam generator. The non-uniform property of local heat transfer with steady flow was examined. The secondary flow and the effect of interaction between the flow oscillation and secondary flow were analyzed on basis of the experimental data. Some new phenomena were observed and explained. Correlations were proposed for average and local heat transfer coefficients both under steady and oscillatory flow conditions. The results showed that there exist considerable variations in local and peripherally time-averaged Nusselt numbers for pulsating flow. Investigations of pressure drop type oscillations and their thresholds for steam-water two-phase flow in a uniformly heated helical tube were also reported.     

Heat transfer and pressure drop in corrugated channels

The convective heat transfer and pressure drop characteristics of flow in corrugated channels have been experimentally investigated. Experiments were performed on channels of uniform wall temperature and of fixed corrugation ratio over a range of Reynolds number, 3220 ≤ Re ≤ 9420. The effects of channel spacing and phase shift variations on heat transfer and pressure drop are discussed. Results of corrugated channels flow showed a significant heat transfer enhancement accompanied by increased pressure drop penalty. The average heat transfer coefficient and pressure drop enhanced by a factor of 2.6 up to 3.2 and 1.9 to 2.6 relative to those for parallel plate channel, respectively, depending upon the spacing and phase shift. The friction factor increased with increasing channel spacing and its phase shift. The effect of spacing variations on heat transfer and friction factor was more pronounced than that of phase shift variation, especially at high Reynolds number. Comparing results of the tested channels by considering the flow area goodness factor (j/f), it was better for corrugated channel with spacing ratio, ? ≤ 3.0 and of phase shift, Ø ≤ 90°. Comparisons of the present data with those available in literature are presented and discussed.     

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.     

Heat transfer and pressure drop characteristics of flow in convergent and divergent ducts

The heat transfer and pressure drop characteristics of the flow in convergent and divergent ducts of rectangular crosssection are obtained through the simulation of the flow by a three-dimensional parabolic model. The results show that in both convergent and divergent flows heat transfer decreases and pressure drop increases sharply near the entrance region of the ducts. Generally, the Nusselt number increases with increasing convergent/divergent angle, aspect ratio, or Reynolds number, and the pressure drop increases with increasing convergent/divergent angle or decreasing aspect ratio or Reynolds number in both flows. However, an increasing convergent/divergent angle may also result in a lower pressure drop owing to the recovery of static pressure from dynamic pressure. Furthermore, the pressure drop in a divergent flow is generally lower than that in a convergent flow except in the entrance region. For divergent flows with high divergent angle or high Reynolds number, flow separation may occur.     

A generalized correlation for evaporation heat transfer of refrigerants in micro-fin tubes

A generalized correlation for flow boiling heat transfer in horizontal micro-fin tubes was developed by implementing non-dimensional parameters accounting for heat transfer enhancement over smooth tubes and physical phenomena into the basic form of a smooth tube correlation. The enhancement factor in nucleate boiling consists of surface tension and turbulence effects generated by the liquid flow. A modified Reynolds number having some similarities with the roughness Reynolds number is introduced into the model to estimate heat transfer enhancement in convective boiling. The ratio of the liquid film thickness to the fin height is also employed in the correlation. The database of the present correlation includes 749 data points for five different refrigerants. The present correlation has a relatively simpler form to employ and yields closer fit to the experimental data with a mean deviation of 20.5% as compared to the existing correlations in the literature.     

Numerical simulation of heat transfer enhancement in wavy microchannel heat sink

In this paper, heat transfer and water flow characteristics in wavy microchannel heat sink (WMCHS) with rectangular cross-section with various wavy amplitudes ranged from 125 to 500 μm is numerically investigated. This investigation covers Reynolds number in the range of 100 to 1000. The three-dimensional steady, laminar flow and heat transfer governing equations are solved using the finite-volume method (FVM). The water flow field and heat transfer phenomena inside the heated wavy microchannels is simulated and the results are compared with the straight microchannels. The effect of using a wavy flow channel on the MCHS thermal performance, the pressure drop, the friction factor, and wall shear stress is reported in this article. It is found that the heat transfer performance of the wavy microchannels is much better than the straight microchannels with the same cross-section. The pressure drop penalty of the wavy microchannels is much smaller than the heat transfer enhancement achievement. Both friction factor and wall shear stress are increased proportionally as the amplitude of wavy microchannels increased.     

Prediction of friction factors and heat transfer coefficients for turbulent flow in corrugated tubes combined with twisted tape inserts. Part 2: heat transfer coefficients

A simple mathematical model following the suggestion of Smithberg and Landis has been created to predict the heat transfer coefficients for the case of a fully developed turbulent flow in a spirally corrugated tube combined with a twisted tape insert. The heat transfer can be predicted from the combined effects of the axial and the tangential boundary layer flows coupled with an additional “vortex mixing” effect near the wall through the solution of the corresponding momentun and energy transfer equations. The “wall roughness” has an effect simultaneously on the axial velocity, secondary fluid motion and the resulting swirl mixing. The model reflects the influence of the “wall roughness” and the twisted tape on the thermal resistances of the helicoidal core flow, twisting boundary layer flow and the viscous sublayer near the wall. The calculated heat transfer coefficients have been compared to 544 experimental points obtained from 57 tubes tested. Four hundred thirty-eight points (80.5%) have a relative difference of less than ±15% and 106 points (19.5%) have a relative difference between ±(15-20)%.     

Exergy transfer effectiveness on heat exchanger for finite pressure drop

In this paper, exergy transfer effectiveness is defined to describe the performance of heat exchangers operating above/below the surrounding temperature with/without finite pressure drop. It is discussed systemically that the effects of heat transfer units number, the ratio of the heat capacity of cold fluids to that of hot fluids and flow patterns on exergy transfer effectiveness of heat exchangers. Furthermore, the results of exergy transfer effectiveness with a finite pressure drop are compared with those without pressure drop when different objective media, such as ideal gas and incompressible liquid, etc. are applied. The detailed comparisons of the exergy transfer effectiveness with heat transfer effectiveness are also performed for the parallel flow, counter flow and cross flow heat exchangers operating above/below the surrounding temperature.