Circular Minichannel Research Articles

Study of the Thermal Effectiveness of Laminar Forced Convection of Nanofluids for Liquid Cooling Applications

Recent theoretical studies show that the convective thermal performance of nanofluids in cooling applications depends crucially on the effective thermophysical properties and, if the performance comparisons are made under different flow constraints, contradictory conclusions can be drawn regarding the effectiveness of the same nanofluid. In this paper, an experimental study is reported on the laminar forced convection of Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> -water and Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> -polyalphaolefin nanofluids through a circular minichannel. With the experimental data, the thermal effectiveness of nanofluids is evaluated using various forms of figure of merit under three typical flow constraints, such as: 1) constant flow rate; 2) constant Reynolds number; and 3) constant pumping power. Although nanofluids enhance convective heat transfer, the results show that their thermal effectiveness is adversely offset by the combined effects of increased viscosity and lower specific heat. In particular, if a convective liquid cooling system is constrained by the constant pumping power condition, there will be essentially no difference in the overall effectiveness between nanofluids and the base fluid when both the thermal and hydrodynamic performances are considered.

Effect of natural convection on conjugate heat transfer characteristics in liquid minichannel during phase change material melting

This article presents numerical and experimental simulation of three-dimensional conjugate heat transfer problem in mini-scaled thermal storage system. The conjugate problem includes melting process of phase change material in the presence of natural convection during laminar flow of heat transfer fluid through circular minichannel. The paraffin wax is used as a phase change material while the water is used as a heat transfer fluid. The main objective of this study is to investigate the effect of the phase change material natural convection during the melting process on the heat transfer fluid thermal characteristics as well as the impact of the natural convection on the melting performance itself. The thermal characteristics are represented by local Nusselt number ( Nu) and local surface temperature. The melting performance is evaluated by fusion time and liquid fraction profile. Two inlet temperatures and velocities of the heat transfer fluid are adopted to highlight the effect of the natural convection. Combination of the inlet temperatures and velocities of the heat transfer fluid forms four cases: case_1 (at [Formula: see text] = 353 °K, [Formula: see text] = 1 m/s), case_2 (at [Formula: see text] = 453 °K, [Formula: see text] = 1 m/s), case_3 (at [Formula: see text] = 353 °K, [Formula: see text] = 0.1 m/s), and case_4 (at [Formula: see text] = 453 °K, [Formula: see text] = 0.1 m/s). Experimental test rig was constructed to verify the computational results and good agreement between both results was achieved. The study shows that the heat transfer fluid encounters an erratic thermal behavior during the phase change material melting process. For example, the local surface temperature experiences dramatic increase and decrease at certain sections of the channel length. The magnitude of this temperature inconsistency interrelates closely to the strength of natural convection impact, and this can expose the minichannel (which has short length) to severe wall thermal stress. The local Nu experiences improvement in some section of the channel and at the same time it suffers from drastic deterioration in its value particularly at the channel end at which the convection current accommodates. The case with the lowest inlet velocity and the highest inlet temperature has the smallest fusion time at expense of the largest heat transfer fluid bulk temperature gradient before reaching the fusion time. The study is considered as a benchmark and helpful guidelines in the design of small-scaled thermal storage systems of phase change material.

Impact of Periodically Generated Hydrodynamic Disturbances on the Condensation Efficiency of R134a Refrigerant in Pipe Mini-Channels

This article discusses an experimental analysis of the influence of periodically generated external hydrodynamic disturbances on the condensation process of the R134 refrigerant in circular pipe mini-channels. The investigations were carried out for horizontal single mini-channels with internal diameters in the range of d = 0.64–3.30 mm. It was established that within the range of frequencies under investigation (f = 0.2–5 Hz), i.e., the frequencies at which disturbances are generated, the disturbances exert an influence on the structure of the characteristic condensation areas in the mini-channel. The disturbances have an unfavorable impact on the reduction of the two-phase length of specific condensation and a reduction of the heat flux in this area. The heat removal intensity in the two-phase area has an impact on the efficiency of the operation of a compact condenser. The impact of more important parameters on this efficiency is discussed.

Investigation of two phase heat transfer and pressure drop of propane in a vertical circular minichannel

This article reports the flow boiling heat transfer and pressure drop results of propane in a vertical circular stainless steel minichannel having an internal diameter of 1.70mm and a heated length of 245mm. Two phase heat transfer and pressure drop experiments have been performed at saturation temperatures of 23, 33 and 43°C. Heat flux is varied from 5 to 280kW/m2 and mass flux is varied from 100 to 500kg/m2s. The results show that the two phase frictional pressure drops, as expected, are increased with the increase of mass flux, vapour qualities and with the decrease of saturation temperature. The heat transfer coefficients are showed to increase with the increase of heat flux and saturation temperature while the influence of mass flux and vapour quality is observed as insignificant. After incipience of dryout, the decrease in heat transfer coefficient and also the two phase frictional pressure drop, especially at higher mass fluxes, is observed. The two phase frictional pressure drop correlations of Müller-Steinhagen and Heck and Friedel and two phase flow heat transfer correlations of Cooper and Liu and Winterton well predicted the experimental results.

Simulation-based analysis of micro-robots swimming at the center and near the wall of circular mini-channels

Swimming micro-robots have great potential in biomedical applications such as targeted drug delivery, medical diagnosis, and destroying blood clots in arteries. Inspired by swimming microorganisms, micro-robots can move in biofluids with helical tails attached to their bodies. In order to design and navigate micro-robots, hydrodynamic characteristics of the flow field must be understood well. This work presents computational fluid dynamics modeling and analysis of the flow due to the motion of micro-robots that consist of magnetic heads and helical tails inside fluid-filled channels akin to bodily conduits; special emphasis is on the effects of the radial position of the robot. Time-averaged velocities, forces, torques, and efficiency of the micro-robots placed in the channels are analyzed as functions of rotation frequency, helical pitch (wavelength) and helical radius (amplitude) of the tail. Results indicate that robots move faster and more efficiently near the wall than at the center of the channel. Forces acting on micro-robots are asymmetrical due to the chirality of the robot’s tail and its motion. Moreover, robots placed near the wall have a different flow pattern around the head when compared to in-center and unbounded swimmers. According to simulation results, time-averaged forward velocity of the robot agrees well with the experimental values measured previously for a robot with almost the same dimensions.

The importance of turbulence during condensation in a horizontal circular minichannel

Three-dimensional simulations of condensation of refrigerant R134a in a horizontal minichannel are presented. Mass fluxes ranging from 50kgm−2s−1 up to 1000kgm−2s−1 are considered in a circular minichannel of 1mm diameter, and uniform wall and vapour–liquid interface temperatures are imposed as boundary conditions. The Volume of Fluid (VOF) method is used to track the vapour–liquid interface; the effects of interfacial shear stress, gravity and surface tension are taken into account. The influence of turbulence in the condensate film is analysed and compared against the assumption of laminar condensate flow by employing different computational approaches and validating the results against experimental data. Under the assumption of laminar condensate flow, experimental heat transfer coefficient values at low mass fluxes can be predicted, but the computed heat transfer coefficient is found to be almost independent of mass flux and vapour quality. Only when turbulence in the condensate film is taken into account does the numerical model capture the influence of mass flux that is observed in the experimental measurements.

Convective mass/heat transfer in the entrance region of the short circular minichannel

The paper presents the experimental results of the investigation of the convective mass transfer near the entrance region of the short minichannel. The experiment was performed using the electrolytic technique – limiting current method. A simple circular channel with a diameter of 1.5mm and a length of 15mm was investigated. The distribution of the mass transfer coefficient at the inner surface of the channel was obtained. Mathematical correlation of the Sh number in dependence of term Re Sc (d/x) was determined for laminar flow: Re numbers were ranging from 110 to 450. The analogous convective heat transfer phenomena were described on the basis of mass/heat transfer analogy. Considerations were related to heat transfer with laminar fluid flow near the hydraulic and the thermal entrance regions of the short circular minichannel. The effects for heat transfer were compared with the theoretical analysis results and the numerical simulation results.

Experimental investigation of dryout of propane in uniformly heated single vertical mini-channels

This article presents dryout results of propane in single vertical circular minichannels made of stainless steel with internal diameters of 1.70 mm and 1.224 mm. The heat flux is increased in steps up to occurrence of dryout. The effects of different parameters such as mass flux, saturation temperature, vapour quality and internal diameter on the dryout are investigated. The results show that the dryout heat flux increases with the increase in mass velocity, with the decrease of vapour quality and with the increase of internal diameter. Almost no effect of saturation temperature on dryout heat flux is observed. Generalised CHF correlations developed for macro and micro scale from the literature are also compared with the experimental results.

Effect of Gravity During Condensation of R134a in a Circular Minichannel

A number of steady-state simulations of condensation of R134a inside a 1 mm i.d. circular minichannel at two far different mass flux values are proposed. The VOF method is used to track the vapour-liquid interface. The first simulations are run at G = 100 kg m − 2 s − 1 and G = 800 kg m − 2 s − 1 assuming that the channel displays horizontal orientation. The effects of interfacial shear stress, gravity and surface tension are all taken into account in this case and the results are validated by means of experimental data already available. As a further step, the same simulations have been run under normal gravity conditions but vertical downflow and finally assuming zero-gravity conditions. The condensation process is found to be gravity dominated at low mass flux, and thus very different results are obtained when neglecting gravity at this mass flux. An opposite result is achieved at high mass flux, as expected from the increased relative importance of interfacial shear stress in this case. The present results also allow to verify the influence of the surface tension effect during condensation in the circular cross section minichannel.

Effect of cross sectional shape during condensation in a single square minichannel

The objective of this work is to present new condensation heat transfer coefficients measured inside a single square cross section minichannel, having a 1.18 mm side length, and compare them to the ones previously measured in a circular minichannel. Tests have been performed with R134a at 40 °C saturation temperature, at mass velocity ranging between 200 and 800 kg m −2 s −1. As compared to the heat transfer coefficients measured in the circular cross section channel, for the same hydraulic diameter, in the square minichannel the authors find a heat transfer enhancement at the lowest value of mass velocity, which must be due to the effect of surface tension. No heat transfer coefficient increase has been found at the highest values of the mass velocity where condensation is shear stress dominated.

Dryout characteristics during flow boiling of R134a in vertical circular minichannels

In this paper, the experimental results of dryout during flow boiling in minichannels are reported and analysed. Experiments were carried out in vertical circular minichannels with internal diameters of 1.22 mm and 1.70 mm and a fixed heated length of 220 mm. R134a was used as working fluid. Mass flux was varied from 50 kg/m 2 s to 600 kg/m 2 s and experiments were performed at two different system pressures corresponding to saturation temperatures of 27 °C and 32 °C. Experimental results show that the dryout heat flux increases with mass flux and decreases with tube diameter while system pressure has no clear effect for the range of experimental conditions covered. Finally, the prediction capabilities of the well known critical heat flux (CHF) correlations are also tested.

Condensation Heat Transfer and Pressure Losses of High- and Low-Pressure Refrigerants Flowing in a Single Circular Minichannel

A 0.96 mm circular minichannel is used to measure both heat transfer coefficients during condensation and two-phase pressure losses of the refrigerants R32 and R245fa. Test runs have been performed at around 40°C saturation temperature, corresponding to 24.8 bar saturation pressure for R32 and 2.5 bar saturation pressure for R245fa. The pressure drop tests have been performed in adiabatic flow conditions, to measure only the pressure losses due to friction. The heat transfer experimental data are compared against predicting models to provide a guideline for the design of minichannel condensers.

Two-phase flow of R-134a refrigerant during flow boiling through a horizontal circular mini-channel

This research focuses on heat transfer to R-134a during flow boiling in a 1.75 mm internal diameter tube. Flow visualisation and heat transfer experiments are conducted to obtain heat transfer coefficients for different flow patterns. The measured data in each flow regime are compared with predictions from a three-zone flow boiling model. The calculations are in fair agreement with the experimental results which correspond in particular to slug flow, throat-annular flow and churn flow regimes under conditions of low heat flux.

Single-Phase Thermal Transport of Nanofluids in a Minichannel

Nanofluids have been proposed as a promising candidate for advanced heat transfer fluids in a variety of important engineering applications ranging from energy storage and electronics cooling to thermal processing of materials. In spite of the extensive studies in the literature, a consensus is lacking on if and how the dispersed nanoparticles alter the thermal transport in convective flows. In this work, an experimental investigation was conducted to study single-phase forced convection of Al2O3-water nanofluid in a circular minichannel with a 1.09 mm inner diameter. The friction factor and convection heat transfer coefficients were measured for nanofluids of various volume concentrations (up to 5%) and were compared with those of the base fluid. The Reynolds number (Re) varied from 600 to 4500, covering the laminar, transition, and early fully developed turbulent regions. It was found that in the laminar region, the nanofluids exhibit pronounced entrance region behaviors possibly due to the flattening of the velocity profile caused by the flow-induced particle migration. Three new observations were made for nanofluids in the transition and turbulent regions: (1) The onset of transition to turbulence is delayed; (2) both the friction factor and the convective heat transfer coefficient are below those of water at the same Re in the transition flow; and (3) once fully developed turbulence is established, the difference in the flow and heat transfer of nanofluids and water will diminish. A simple scaling analysis was used to show that these behaviors may be attributed to the variation in the relative size of the nanoparticle with respect to the turbulent microscales at different Re. The results from this work suggest that the particle-fluid interaction has a significant impact on the flow physics of nanofluids, especially in the transition and turbulent regions. Consequently, as a heat transfer fluid, nanofluids should be used in either the laminar flow or the fully developed turbulent flow at sufficiently high Re in order to yield enhanced heat transfer performance.

Flow pattern and heat transfer characteristics of R-134a refrigerant during flow boiling in a horizontal circular mini-channel

Flow boiling heat transfer of R-134a refrigerant in a circular mini-channel, 600 mm long with a diameter of 1.75 mm, is investigated experimentally in this study. The test section is a stainless steel tube placed horizontally. Flow pattern and heat transfer coefficient data are obtained for a mass flux range of 200–1000 kg/m 2 s, a heat flux range of 1–83 kW/m 2 and saturation pressures of 8, 10, and 13 bar. Five different flow patterns including slug flow, throat-annular flow, churn flow, annular flow and annular-rivulet flow are observed and the heat transfer coefficient data for different flow patterns are presented. The heat transfer coefficient increases with increasing heat flux but is mostly independent of mass flux and vapour quality. In addition, it is indicated from the experiments that the higher the saturation pressure, the lower is the heat transfer coefficient. Comparisons of the present data with the existing correlations are also presented.

Experimental study on condensation heat transfer inside a single circular minichannel

Abstract The measurement of the condensation heat transfer coefficient inside micro- and minichannels is still somewhat elusive due to the difficult task of getting accurate values of the heat transfer coefficients during the condensation process, particularly when studied within single minichannels. The present paper reports local heat transfer coefficients obtained from the measurement of the local heat flux and the direct measurement of the saturation and wall temperatures during condensation of R134a and R32 within a single circular 0.96 mm diameter minichannel. Except for the lowest mass velocity, the test results do not show significant discrepancy from the trends expected for macroscale tubes.

Characteristics of R113 flow boiling heat transfer in non-circular minichannels

As only few reports on flow boiling heat transfer in non-circular minichannels are published,flow boiling heat transfer characteristics of R113 in square-and triangular-section minichannels are experimentally investigated.The experiments are performed with the inlet vapor quality ranging from super-cooling to 1.0,mass velocity from 400 to 3 300 kg/(m2.s),and heat flux from 20 to 150 kW/m2.And the experimental data are compared and analyzed with those in circular minichannel hav-ing close hydrodynamics diameter.The results show that thelocal wall temperature depends heavily on the mass velocity in non-circular channel flow boiling,which is affected by the heat flux and the mechanism of flow boiling heat transfer.Both mass velocity and heat flux could effect on the mean heat transfer coefficient of saturated flow boiling in non-circular minichan-nels.According to the experimental data in both non-circular and circular minichannel with nearly equal hydrodynamic di-ameter,the non-circular minichannels appear to have remark-able heat transfer enhancement effect.

Experimental investigation and modelling of heat transfer during convective boiling in a minichannel

Flow boiling heat transfer characteristics of water are experimentally studied in a circular minichannel with an inner diameter of 1500 μm. The fluid flows upwards and the test section, made of the nickel alloy Inconel 600, is directly electrically heated. Thus, the evaporation takes place under the defined boundary condition of constant heat flux. Mass fluxes between 50 and 100 kg/(m 2 s) and heat fluxes from 10 to 115 kW/m 2 at an inlet pressure of 3 bar are examined. Infrared thermography is applied to measure the outer wall temperatures of the minichannel. This experimental method permits the identification of different boiling regions, boiling mechanisms and the determination of local heat transfer coefficients. Measurements are carried out in single-phase flow, subcooled and saturated boiling regions. The experimental heat transfer coefficients in the region of saturated boiling are compared with correlations available in literature and with a physically founded model developed for convective boiling.

A study on performance analysis of the helically coiled evaporator with circular minichannels

In order to develop a compact evaporator, experiments that show characteristics of evaporating heat transfer and pressure drop in the helically coiled minichannel were performed in our previous research. This study was focused on the performance analysis of helically coiled heat exchangers with circular minichannels with an inner diameter=1.0 mm. The working fluid was R-22, and the properties of R-22 were estimated using the REFPROP program. Numerical simulation was performed to compare results with the experimental results of the helically coiled heat exchanger. As the heat transfer rate and pressure drop were calculated at the micro segment of the branch channels, the performance of the evaporator was evaluated. The following conclusions were obtained through the numerical simulations of the helically coiled heat exchanger. It showed good performance when the flow rate of each branch channels was suitable to heat load of air-side. The numerical simulation value agreed with experimental results within ± 15%. In this study, a numerical simulation program was developed to estimate the performance of a helically coiled evaporator. And, an optimum helically coiled minichannels evaporator was designed.

Effects of Geometrical and Thermophysical Parameters on Heat Transfer Measurements in Small-Diameter Channels

An experimental investigation of the liquid flow friction factor and heat transfer coefficient in small diameter channels is presented. Rectangular and circular mini-channels with hydraulic diameters from 0.77 to 2.01 mm were used. Literature in the 1990s showed scattered experimental results and concluded that new physical phenomena in mini-channels could occur. However, the present experimental results show that three main causes can explain how different results were obtained in mini-channels compared to macro-tubes: uncertainties on the channel dimensions, inlet and outlet singular pressure losses, and longitudinal heat conduction. When these elements are taken into account, good agreement is found with standard correlations or theories.