Heat Transfer Coefficient In Channel Research Articles

Experimental study on single-phase convective heat transfer of interlocking double-layer counterflow mini-channel heat sink

This study proposes an interlocking double-layer mini-channel heat sink that can realize counterflow in which the flow direction is opposite in the adjacent channel. The heat sink consists of two-channel layers and a header/plenum structure serving as a pre-flow path for counterflow into the channels. The novelty and significance of the current study are that the counterflow multi-channel heat sink was first experimentally implemented through the design of a new internal flow distribution structure. The test data are obtained from the experimental evaluation of the single-phase flow cooling performance of the heat sink. The heat transfer characteristics of the heat sink are analyzed by separating the change in the single channel heat transfer coefficient increased by the counterflow channel effect. The thermal resistance representing the overall cooling performance of the proposed heat sink shows a significantly small value of 10-4 K-m2/W order, which indicates the heat transfer performance advantage of the new heat sink. The beneficial effect of counterflow is verified by comparing the heat transfer coefficient obtained from the proposed heat sink and the prediction using the previous correlation for unidirectional single-phase flow heat transfer coefficient. The measured heat transfer coefficients are much larger than the predicted values by all kinds of past correlations. The difference between the measured value and the predicted value is much greater at low flow conditions where the temperature gradient increases more significantly with the flow direction. It can be concluded that this indirectly implies the fact that temperature gradient mitigation by counterflow results in an additional heat transfer improvement that is unpredictable in the previous unidirectional flow correlations. A new correlation is proposed to predict the heat transfer coefficient affected by the counterflow. This study is the first to experimentally implement a counterflow heat sink, confirming the distinct advantage of improved cooling performance over conventional heatsinks. In addition, this study is meaningful in that it presents a performance prediction model that can be used in design for the practical application of counterflow heat sinks.

Research on Flow and Condensation Heat Transfer Coefficient of multi-channel cylinder dryer in U-shaped Section

The cylinder dryer is the most energy-consuming part of the papermaking drying section. Whether the drying cylinder can effectively drain the drying cylinder affects the drying efficiency of the drying cylinder. The design of the multi-channel drying cylinder effectively improves the problem of the difficulty of the water accumulation discharge. In order to obtain a channel cross-sectional shape with better drainage and better heat transfer, two channels with different cross-sectional shapes are selected to study the flow and heat transfer of steam in this experiment. The changes of the channel heat transfer coefficient under different working conditions are studied and compared. The result show that the U-shaped channel heat transfer effect is better than that of other shape channels in the multi-channel dryer.

Numerical Study of Single-Phase Heat Transfer Performance of a Mini/Micro-Channel Integrated With Multiple Bypass Micro-Nozzles

Abstract Surface temperature uniformity is an important factor in the thermal management of electronics. The present numerical study investigates the influence of multiple bypass injections on the wall temperature distribution of a single-phase mini/micro-channel. The proposed scheme consists of sending a fraction of the coolant through the inlet of a 0.6 mm deep, 2.5 mm wide, and 25 mm long channel and injecting the remaining coolant through multiple bypass inlets on top of the channel positioned at different axial locations. The study explores four different configurations: the first one being three equispaced bypass micro-nozzles or bypass inlets of uniform diameter (1 mm), the second one being three equispaced bypass micro-nozzles of varying diameter (2 mm, 1 mm, and 0.5 mm), the third one being five equispaced bypass micro-nozzles of varying diameter (2 mm, 1 mm, 0.5 mm, 0.25 mm, and 0.125 mm), and the fourth one being five bypass micro-nozzles, but with three equispaced bypass inlets of varying diameter (2 mm, 1 mm, and 0.5 mm), and the last two bypass inlets of the same diameter as that of the third inlet (0.5 mm). Water is considered as the coolant in the study and the simulations are carried out for two mass fluxes of 465 kg/m2s and 930 kg/m2s and two heat fluxes of 25 kW/m2 and 125 kW/m2. The thermal performance of the channel is evaluated for bypass percentages of 25%, 50%, and 75%, with the Reynolds number varying from 150 to 900 at the primary channel inlet and at the secondary channel inlet, and the range of the nozzle Reynolds number varying from 10 to 707. The fourth configuration results in a near uniform wall temperature distribution, with 82–89% reduction in the wall temperature nonuniformity compared with the no-bypass case. The reductions for the third, second and first configurations are 65–71%, 53–76%, and 54–74%, respectively. The third configuration results in an average heat transfer coefficient enhancement of up to 49%. On the whole, the improvement in the wall temperature uniformity is higher than the increase in the pressure drop, and the increase in the channel heat transfer coefficient is higher than pressure drop for some cases.

Analysis of flow pattern change in horizontal mini-channel under electric field force

In this paper, the phenomenon of the flow pattern in the channel changes when the electric field force exists. Through the visual observation in the experiment, it is found that the study found that the bubble behavior was suppressed by the electric field force. When the electric field is large enough, the working fluid in the channel will always maintain a single-phase liquid flow. However, When there is enough superheat at the bottom of the mini-channel, the liquid film at the bottom of the liquid phase changes to a vapor phase, and the dryout appears in the channel, and it rapidly changes into a mist flow as the heating power increases. This phenomenon of flow pattern change due to electric field force is referred to herein as a flow pattern special phenomenon. Due to the existence of this special flow phenomenon, the temperature difference between the inlet and outlet of the working fluid in the channel has a large change. Through calculation, it is found that the dimensionless temperature difference between the inlet and outlet at a voltage of 200 V is 1.67 times that of 0 V. At the same time, the channel heat transfer coefficient increases with the increase of voltage.

Wall Heat Flux Evaluation in Regeneratively Cooled Rocket Thrust Chambers

In the present Paper, different wall heat flux evaluation methods for rocket engines are analyzed and compared. The test case for the comparison is a supercritical load point of an upper-stage engine. The evaluation of the heat flux using existing gradient methods shows unacceptable deviations, whereas significant improvements are found with an inverse heat transfer method. Using a Nusselt number correlation for the modeling of the cooling channel heat transfer coefficient, good agreement with the experimental profiles is achieved, while the number of installed thermocouples is held at a minimum. However, the choice of the Nusselt number correlation directly influences the obtained results and strongly depends on the fuel and geometry of the system. Using a simultaneous optimization of the heat flux and heat transfer coefficient, on the other hand, seems to eliminate the need for additional modeling and leads to great agreement with the experiment. A new optimization method proposed in this Paper based on a preprocessed Jacobi matrix significantly speeds up the estimation of the free parameters. With the obtained results, the placement of the thermocouples before the design of the hardware can be optimized, leading to reduction of manufacturing costs.

Mitigation of heat transfer deterioration in a circular tube with supercritical CO2 using a novel small-scale multiple vortex generator

Abstracts This paper explores the performance effects of a novel small-scale multiple vortex generators (MVG) on heat transfer deterioration (HTD) mitigation and pressure drop using supercritical CO2 (sCO2) as the working fluid. The HTD mitigation and pressure drop are examined using an overall performance factor, R = ΔHTD/(Δp/Δp0). Numerical calculations are first conducted to investigate the effects of single conventional vortex generator's (VG) configurations (rectangular, triangular and trapezoidal). The results indicate that under the same aspect ratio, rectangular VG has the largest area facing the fluid flow and the largest angle of attack which induces the strongest longitudinal vortices leading to its highest value of R. Then, the morphological properties (density and height) of five pairs of evenly distributed rectangular VGs in an MVG are varied while their uniformity is maintained. With the increase in MVG's height, the channel's heat transfer coefficient (HTC) profiles increase significantly at the locations where MVG is installed while the opposite trend is found for R. The density and nonuniformity of the MVG have the highest performance effects on HTD mitigation, with R of more than 18% and 11% for the density of 2.5 and non-uniform MVG respectively. Analysis reveals that as the density of MVG increases, the fluid recirculation zone behind each VG diminishes correspondingly, causing a greater vortex intensity mixing by swirling flows which enhances the downstream production of turbulence kinetic energy (TKE), thus weakening the buoyancy forces and leading to HTD mitigations. In addition, by further increasing the MVG density beyond 2.5, it was found that the HTC decreases while the pressure drop increases, with the wall temperature peaks stabilizing at 178 °C, which demonstrates the breakdown of Reynold's analogy. Increasing the number of VGs in an MVG increases its delay and broadening effects on the wall temperature peaks by further delaying the boundary layer recovery caused by interaction of longitudinal vortices generated by each VG, but has relatively little effects on R. Also, analysis further reveals the presence of buoyancy-induced flow oscillations near the channel walls, which are attenuated by the VG and MVG effects. Overall, MVG offers superior performance than convectional single VGs in terms of HTD mitigation, and the results presented here can be employed as a reference guide in the design of highly efficient, safe and reliable supercritical heat exchanger systems.

A Mathematical Model of Local and Average Heat Transfer in Channels with Heat Transfer Intensifiers

The expressions derived previously from the three-layer Owen model of a turbulent boundary layer and the Deissler and the van Driest models are demonstrated to be valid for predicting heat transfer coefficients in the entrance region of channels. The basic parameters in these expressions are the dynamic velocity, dimensionless viscous sublayer thickness, and dimensionless turbulent boundary layer thickness. Correlations are presented for predicting these parameters in the entrance region of a plate or circular pipe. The predicted heat transfer coefficients agree with the available data. With consideration of the fact that the friction and the heat transfer laws established by S.S. Kutateladze and A.I. Leont’ev are conservative with respect to disturbances in the turbulent boundary layer, parameters of the expressions for average heat transfer coefficients in channels with heat transfer intensifiers (such as roughness, transverse ring protrusions, and annular groves in pipe walls) were determined. In calculating friction and heat transfer, the main characteristics are the dynamic velocity and the ratio of the friction coefficient on a smooth surface to that on a surface with heat transfer intensifiers. The expression was derived for calculating the Nusselt number as a function of the Reynolds number and the friction factor. The predicted average heat transfer coefficients agree well with the experimental data and the predictions by empirical correlations. Calculations were performed in a wide range of Reynolds number (from 104 to 106) and smooth-to-intensified surface friction factor ratio (from 1.92 to 9.2). The proposed correlations can be used for predicting local heat transfer coefficients in the entrance region on a body in a flow and average heat transfer coefficients in intensified channels.

Two-Phase Flow In Mini-Scale Complex Geometry

Heat-transfer coefficients are reported for one surface, a plain surface, with 50 mm square base area. Parallel channel test piece has one mm by one mm, 25 channelsThe data were produced while boiling R113 at atmospheric pressure. For this surface, the mass flux range was 200 – 600 kg/m2s and the heat flux range was 5 - 80 kW/m2. The results obtained have been compared with standard correlations for tube bundles. The measured heat-transfer coefficients for the parallel micro-channel surface are slightly bigger for any plate channel surface. It is dependent on heat flux and reasonably independent of mass flux and vapor quality. Thus, heat transfer is probably dominated by nucleate boiling. The parallel channel heat transfer coefficients were typically bigger than other plate -channel values.

Numerical simulation of convective heat transfer coefficient in channel with corrugated walls

The present work is a contribution to study of convective heat transfer coefficient inside a rectangular channel with corrugated walls. Triangular, square and rectangular shaped configurations were studied for a range of geometric parameters during simulation. The Navier-Stokes equations were numerically solved using the finite volume method through the Easy CFD_G package code in its V.4.1.0 version. With prescribed temperatures and velocities, the model predicts the behavior of the airflow inside the device. The temperature and velocity distributions are first predicted. From these distributions, the convective heat transfer coefficients along the surface of the objects placed inside the system are determined. Also, from the pressure distribution, the pressure drops along the channel are predicted. The results show that the triangular corrugated-shaped configuration with h = 5 [cm] and α = β = 60° enable to obtain the best value of convective heat transfer coefficient on the surface of the objects which is 2.70 [Wm-2°C-1] resulting in a pressure drop of 0.11 [Pa], while for parallel-plate channel configuration this same coefficient is 1.12 [Wm-2°C-1]. The energy balance enabled to conclude that the energy gain by convection air/objects is superior to the air pump energy to overcome the pressure drop.

Dominant dimensionless groups controlling heat transfer coefficient during flow condensation inside pipes

A study of the dominant dimensionless groups describing flow condensation at high mass fluxes inside circular pipes was performed by gathering experimental data from the literature covering a wide range of pipe diameters and fluid properties. A new model is presented based on an equivalent Reynolds number defined as the sum of the superficial liquid and vapour Reynolds numbers and an equivalent Prandtl number given as the sum of the liquid and vapour Prandtl number weighted with the thermodynamic quality. This simple model is able to capture the heat transfer coefficient of channels from a hydraulic diameter of 67μm up to pipes of 14.45mm in diameter for several fluids.

Saturated flow boiling heat transfer investigation for nanofluid in minichannel

In the present paper, a theoretical saturated flow boiling heat transfer coefficient correlation for nanofluid in minichannel has been proposed based on the contributions of convective boiling and nucleate boiling. For convective heat transfer, the absorbed heat amount of nanoparticles from fluid has been deduced by the fractal distribution theory. For nucleate boiling, the probability density equation of bubbles random growth is obtained on the basis of a Markov process. As a result, the nucleate flow boiling heat transfer coefficient formula is derived. In addition, three mass fractions for 0.2%, 0.5% and 1.0% Al2O3/H2O nanofluid are prepared by ultrasonic vibration and saturated flow boiling heat transfer coefficients in different rectangular minichannels with cross section 0.3×2mm, 0.6×2mm, 2×2mm are experimentally investigated. The results show that the heat transfer performance is enhanced after adding nanoparticles in base fluid and the average saturated flow boiling heat transfer coefficients of 0.2wt.%, 0.5wt.% and 1.0wt.% Al2O3/H2O nanofluid respectively increase by 11.2%, 15.4% and 18.7% compared with that of deionized water. Meanwhile, the average saturated flow boiling heat transfer coefficients for 1.0wt.% Al3O2/H2O nanofluid are increased by 21.1%, 26.2% in channels with cross section 0.3mm×2mm, 0.6mm×2mm compared with the average heat transfer coefficients in channels with cross section 2mm×2mm, which illustrates that the capillary effects are strengthened with the hydraulic diameter decreasing. Furthermore, a comparison is made between the experimental and theoretical data. The results show that the theoretical heat transfer model has a good universal prediction for nanofluid saturated flow boiling heat transfer in minichannel.

Heat transfer and hydraulic resistance in channels with spherical protrusions

This article presents an analytical overview of experimental studies of hydraulic resistance and heat transfer in channels with disturbed flow surfaces in the form of systems of spherical protrusions in a wide range of flow and design parameters. A uniform approach to generalization of experimental data by various authors has been developed considering differences in the geometry of heat-transfer surfaces. Uniform universal generalizing dependences have been obtained for the hydraulic resistance and heat transfer coefficients in channels with spherical protrusions.

Possibilities of the Use of the Electrolytic Technique for the Investigations of Mass/Heat Transfer in Nanofluid

In the paper the author presents the possibilities of the application of the electrolytic technique for the investigation of heat transfer coefficients in channels with nanofluids. The electrolytic technique called the limiting current method enables to obtain mass transfer coefficients on the basis of the electrochemical processes and the laws governing these physical phenomena. The exemplary graph presenting the limiting currents values resulting from the experiment is shown in Fig. 1. These are the voltammograms at different Reynolds numbers and the ion concentration Cb. Heat transfer coefficients can be calculated using the correlations describing the analogy of mass and heat transfer processes. Some cases of the possible application of the electrolytic technique and factors influencing the mass transfer experiment including the state of the electroactive surface, change in ion concentration and thickness of the diffusion layer are discussed in [1]. In this work the author focused on the mass transfer experiment with the application of the nanofluid. Special properties of the nanofluid should be taken into account in the investigations. The paper presents the preliminary results of the experiment with nanofluid in the annular channel.

Conjugate Heat Transfer High-Heat-Flux Amplification Simulation

High-heat-flux (HHF) removal (HHFR) limits can be formidable technological barriers that prevent or limit the normal implementation or optimization of new and novel devices or processes. A conjugate heat transfer HHFR simulation methodology has been developed with excellent resulting accuracy (>98.0% accurate) for predicting HHF amplification (peaking factors) and the peak flow channel inside wall temperature. The methodology can be used directly or expanded to a correlation form. Although the simulation utilized axial and swirl water flows with single-phase fully developed turbulent and subcooled flow boiling in a single-side-heated circular inside flow channel with a rectangular outer boundary, the methodology appears to be fluid- and flow regime-independent (e.g., applicable to developing or jet impingement flows) so that other fluids (e.g., gases, dielectric liquids, liquid metals) and flow regimes can be employed possibly for HHFR applications requiring specialized fluids and/or flow conditions. However, more work is required to validate the applicability of this methodology (and the correlation) to other fluids, flow regimes, and channel materials. Further, the approach can be expanded possibly to include applications employing a hypervapotron for HHFR. For the prototypic simulation cases (≤38.0 MW/m2) considered, the circumferential inside flow channel heat transfer coefficient distribution [h([varphi])] was not known a priori, so, h([varphi]) was determined from the unknown local inside wall heat flux via iterative finite element conjugate heat transfer analyses for flow regimes ranging from fully developed turbulent subcooled flow boiling (at the top of the flow channel) to single-phase turbulent flow (at the bottom of the flow channel).

Heat transfer in rectangular channels with transverse and V-shaped broken ribs

Repeated ribs are used on heat exchange surfaces to promote turbulence and enhance convective heat transfer. Applications include fuel rods of gas-cooled nuclear reactors, inside cavities of turbine blades, and internal surfaces of pipes used in heat exchangers. Despite the great number of literature papers, only few experimental data concern detailed distribution of the heat transfer coefficient in channels with rib turbulators. This issue was tackled by means of the steady-state liquid crystal thermography: a pre-packaged liquid crystal film was glued onto the heated surface, and the colour map was taken by a video camera at the steady state of a given experiment. After calibration tests to assess the colour–temperature relationship had been performed, local heat transfer coefficients were obtained by applying custom-made software to process the digitised colour images. Liquid crystal thermography was applied to the study of heat transfer from a rectangular channel (width-to-height ratio equal to five) having one surface heated at uniform heat flux and roughened by repeated ribs. The ribs, having rectangular or square sections, were deployed transverse to the main direction of flow or V-shaped with an angle of 45 or 60 deg relative to flow direction. The effect of continuous and broken ribs was also considered. Local heat transfer coefficients were obtained at various Reynolds numbers, within the turbulent flow regime. Area-averaged data were calculated in order to compare the overall performance of the tested ribbed surfaces and to evaluate the degree of heat transfer enhancement induced by the ribs with respect to the smooth channel.

Subcooled Water Flow Boiling Experiments under Uniform High Heat Flux Conditions

This work involves steady-state high heat flux removal in fusion reactor beam dumps, first walls in compact fusion reactors, and other applications with smooth surfaces and/or irregular coolant channel cross sections. In such applications, the coolant pressure is required to be low (/approx/ 1.0 MPa), and the coolant channels are moderately long (length-to-diameter ratio (L/D) /approx/ 100). The present experiments have resulted in high heat flux data in a region where only sparse data existed. Subcooled flow boiling measurements were performed for the critical heat flux (CHF), local (axial) variations of the coolant channel's heat transfer coefficients, and pressure drop for horizontal, uniformly heated tubes. The tubes had inside diameters of 0.3 cm, a heated L/D ratio of 96.6, and were made of amzirc (zirconium-copper). The coolant was degassed, deionized water. The exit pressure and the inlet water temperature were held approximately constant at 0.77 MPa and 20/sup 0/C, respectively. From experiment to experiment, the inlet temperature varied slightly (+- 1.5/sup 0/C) from 20/sup 0/C. The actual measured inlet temperature was used in reducing the experimental data. Measurements of the above quantities were performed for the mass velocity and exit subcooling, varying from 4.6 to 40.6 Mg/m/sup 2/ . smore » and 30 to 74/sup 0/C, respectively. For these ranges, (a) the subcooled flow boiling CHF varied from 625 to 4158 W/cm/sup 2/, (b) the heat transfer coefficients during fully developed nucleate boiling varied from 30 to 400 kW/m/sup 2/ . K (Nusselt number = 150 to 1700), and (c) the overall pressure drop varied from -- 1.75 to 0.9 times the adiabatic pressure drop. For the flow conditions and geometry parameters given above, least-squares equations for the CHF were developed in terms of both the liquid Reynolds number and the exit subcooling. The average percent deviation of the developed equations from the data was <12.5%.« less

Influence of Oscillation-Induced Diffusion on Heat Transfer in a Uniformly Heated Channel

An analysis of the effect of flow oscillations on laminar flow heat transfer in a channel with uniform heat addition is presented. It is shown that the effect of flow oscillations will be to reduce the channel heat transfer coefficient. This effect is due to the fact that the heat addition along the channel wall produces an increasing fluid temperature along the channel length. The flow oscillations interacting with this positive temperature gradient will induce a heat flow back toward the channel inlet. This will tend to inhibit the heat transfer process and will raise the wall temperature required to transfer away a given amount of heat at the channel wall.