Channel Heat Research Articles

Thermofluidic analysis of Al2O3-water nanofluid cooled branched wavy heat sink

The thermofluidic analysis of Al2O3-water nanofluid-cooled, branched wavy heat sink microchannel (BWHS MC) was carried out for Re = 100–300. The conjugate heat transfer was calculated using ANSYS fluent. The fluid flow is simulated using the RNG k-ε model in the full domain numerical analysis. The phenomenological behaviour of the nanofluid flow through BWHS MC is further studied using numerical results. In comparison, experiments were also performed using a straight channel heat sink microchannel (SCHS MC) of the same hydraulic diameter as the BWHS. The branched wavy microchannel encouraged the secondary flow, thus enhanced the macroscopic mixing. Apart from disruption of the boundary layer and its reinitialization, vortices have been formed near the secondary channel, which improves thermal performance. BWHS reported a higher pressure penalty than SCHS, but the BWHS showed superior performance over SCHS. The heat transfer coefficient is increased at increasing nanofluids concentrations for any given Reynolds number. For the BWHS MC, an enhancement of about 154% of the heat transfer coefficient with a 2% volumetric concentration of nanofluids is achieved at Re = 300 compared to the SCHS MC at similar conditions.

Computational analysis of a heat transfer characteristic of a wavy and corrugated channel

In a compact heat exchanger, optimal surface area utilization through the use of various channel and surface configurations contributes to creating a more efficient heat exchanger. In recent years, researchers have focused their attention on the usage of various types of channels and surfaces in heat exchangers, and a number of performance-based assessments have been published in this field. An in-depth thermal analysis of fluid flow and heat transfer within a wavy channel and corrugated channel heat exchanger is presented in this paper. Considering problem statement and the energy equation a two-dimensional computational model of the wavy and corrugated channel has been designed with the aim of simulation. The Navier-Stokes equation, which governs the flow, is solved with the help of the ANSYS fluent solver. Through the whole length of the channel, the effect of Reynolds number on various parameters such as surface Nusselt number, friction factor, wall shear, and pressure distribution has been explored to determine their relationships. According to the findings, the wavy channel has a better heat transfer capability than the smooth and corrugated channels when subjected to turbulent flow (with a high Reynolds number).

Robust and general predictive models for condensation heat transfer inside conventional and mini/micro channel heat exchangers

To design, scale-up, and optimize the two-phase flow heat exchangers, accurate information about the heat transfer coefficient (HTC) is required. Here, an extensive database including 11,128 experimental data points from 80 sources, covering 37 condensing fluids over a wide range of operating conditions, were gathered for developing general and accurate models to forecast the HTC in mini/micro and conventional channel heat exchangers. The conventional intelligent approaches, namely, gaussian process regression (GPR), radial basis function (RBF), and hybrid radial basis function (HRBF) and also the least square fitting method (LSFM) were utilized for development of the new models. Firstly, the gathered data were used in evaluation of earlier two-phase HTC models, and these models showed average absolute relative error (AARE) values higher than 29%. Thus, more accurate models are necessary for these systems. Based on Pearson’s correlation analysis, eight dimensionless groups were selected as the most important input parameters. The GPR model showed the best results in estimation of Nu number for the test process with AARE of 4.50%. However, RBF and HRBF models estimated the HTC with AARE values 19.41% and 24.36% for testing data points, respectively. In addition, a general explicit correlation was developed by the least square fitting method (LSFM) for estimating the HTC, with acceptable results at an AARE of 23.40% for all analyzed data. The performance of the new general correlation, GPR and the previous models were assessed for different channel sizes, flow regimes, fluid types, and operating conditions. The new models and well-known earlier ones were examined with about 372 additional data samples from four new independent sources. The proposed models showed excellent results for prediction of HTC. Finally, a sensitivity analysis was studied based on the experimental data and the outcomes of the LSFM.

Hydrothermal performance evaluation of super hydrophobic square pin fin mini channel heat sink

Efficient heat removal with less pressure drop is the focal point of research work in the field of thermal management systems. This research work is aimed to analyze inline square pin fin mini channel heat sink with superhydrophobic coating. The SiO2 aqueous nanofluids with 0.01% and 0.02% volumetric concentration are used as heat transfer fluid. Data is attained at steady-state with a power input of 40 W, 55 W, and 70 W, flow rate of 300-700 ml per minute, and Reynolds number ranging from 400-1230. The test rig is authenticated by matching data of distilled water for a simple pin fin heat sink with the theoretical model of Tullius which matched the data well. Superhydrophobic mini channel heat sink gave a better performance with 25.23%, 21.8%, and 23.38% augmentation in Nusselt number and 33.19%, 30.5%, 31.1% reduction in pressure drop for distilled water, SiO2 (0.01%) and SiO2 (0.02%), respectively, as compared to the conventional pin fin mini channel heat sink. The nutshell of this experimental work is magnification in heat transfer with a reduction in pressure drop.

NUMERICAL INVESTIGATION OF COOLING BEHAVIOR IN MINICHANNEL HEAT SINK OF MULTIPLE CONFIGURATIONS

NUMERICAL INVESTIGATION OF COOLING BEHAVIOR IN MINICHANNEL HEAT SINK OF MULTIPLE CONFIGURATIONS

Modeling heat and mass transfer processes during evaporation of a water film in a horizontal channel with a cocurrent flow of moist air

Modeling of heat and mass transfer processes in a horizontal channel during evaporative cooling of a moist air flow with regard to the finite thickness of the liquid film is considered. The mathematical model consists of a system of differential equations in the boundary layer approximation. The simulation results have been obtained in a wide range of initial parameters: temperature T 0 = 10÷50°C, humidity φ0=0÷100%, Reynolds number Re=100÷2000. Calculations were carried out at atmospheric pressure. Quantitative analysis of influence of initial parameters of flows on values of parameters of wet air flow at the outlet of the channel with and without taking into account the final thickness of the water film was carried out.

Expansion characteristics of a discharge column in an electrode–ice gap

When modeling the propagation of an electric discharge on an ice-covered insulator, the diameter and temperature of the discharge column are vitally important. This paper studies the thermal expansion characteristics of a discharge column occurring after electrode–ice gap breakdown both experimentally and theoretically. It is found that the air part of the discharge channel expands faster and has a higher temperature and larger diameter than the surface part. To provide a quantitative explanation of this phenomenon, a physical model is established based on the gap breakdown process, discharge channel heat conduction, and molecular thermal relaxation theory. The results of theoretical calculations show that the experimentally observed differences in discharge column thermal expansion characteristics result from the heating effect of brush discharge and from differences in heat conduction around the column.

Saturated flow boiling inside conventional and mini/micro channels: A new general model for frictional pressure drop using genetic programming

• A new general model for FPD during saturated boiling was proposed using GP. • 6021 experimental data samples from 42 published sources were analyzed. • Present model showed favor outcomes for different flow regimes and channel sizes. • The accuracy of previous models was considerably lower than the present one. • The new model presented favor physical trends at different operating conditions. This study presents a general explicit model for estimating the saturated flow boiling frictional pressure drop (FPD) in conventional (macro) and mini/micro channels heat exchangers. An extensive database including 6021 experimental data samples has been gathered from 42 published sources, covering a broad range of fluids, channel diameters and operating parameters. The new model is based on the separated model suggested by Lockhart and Martinelli (1949) for two-phase flow. Thus, the two-phase multiplier, ϕ l o 2 has been estimated using the intelligent approach of genetic programming (GP). The presented model predicts the mentioned database with a reasonable value of average absolute relative deviation (AARD) of 21.34%. Moreover, 74.85% of predicted data have an error of lower than 30% of the experimental values. The entire database is compared with ten well-known two-phase pressure drop correlations for the evaluation of previous models. But all of them showed a total AARD of more than 27%. The GP model shows good accuracy for both conventional and mini/micro channels and different flow regimes, including low and high Reynolds numbers. In addition, it is applicable for estimating the boiling FPD in different operating conditions. Based on 752 additional data from 4 independent sources, the new model provides the best predictions for estimating the FPD in conventional and mini/micro channels.

Experimental thermal and hydraulic study of super hydrophobic wavy mini channel heat sink using aqueous nanofluids

Super hydrophobic wavy mini channel heat sink is analyzed experimentally for thermal and hydraulic analysis using SiO2 and TiO2 aqueous nanofluids with volumetric concentration of 0.01% and 0.02%. The test rig is validated by comparing the data obtained for water (distilled) using simple wavy mini channel heat sink (without super hydrophobic coating) with the theoretical model of Peng and Peterson. Three cartridge heaters were used to supply uniform heat flux to the base of mini channel heat sink. Results were obtained at steady state in laminar regime of flow at Reynolds number ranging from 400 to 1230. Data were acquired at heating powers of 40, 55, and 70 W. Augmentation of 31.37% in heat transfer was recorded for super hydrophobic mini channel heat sink as compared to conventional heat sink with a reduction of 39.02% in pressure drop. The results can be summarized as enhancement of heat transfer with a noticeable diminution in pressure drop and hence reduced power consumption.

Evaluation Model for Fast Convective Heat Transfer Characteristics of Thermal Cracked Hydrocarbon Fuel Regenerative Cooling Channel in Hydrocarbon-Fueled Scramjet

• A fast flow and heat transfer evaluation model with simplified endothermic pyrolysis was established. • Influence of inlet fuel mass flow on flow and heat transfer characteristics was studied. • Influence of two types heat flux on flow and heat transfer characteristics was studied. • An inner fuel temperature inside cooling channel detective method was derived from above results. Regenerative cooling is critical for hypersonic propulsion systems, wherein the flow and heat transfer characteristics of the hydrocarbon fuel in the cooling channel are crucial. The most commonly used existing evaluation models for flow and heat transfer characteristics use one-step overall reaction endothermic pyrolysis; however, they are limited to mildly cracked hydrocarbon fuel working conditions. To quickly evaluate the flow and heat transfer characteristics of hydrocarbon fuel under both mild- and heavy-cracking conditions, a quasi-one-dimensional model was established in this study; considering 18 species and 24 reactions, and a simplified mechanism of n-decane. The flow and heat transfer characteristics under different cooling channel lengths and heat fluxes were studied. The results show that the proposed model has fast flow and heat transfer characteristics evaluation capability, and small persistent errors. Furthermore, the calculation results under different inlet fuel mass flow and heat flux working conditions showed that the hydrocarbon fuel flow and heat transfer characteristics vary with the fuel mass flow and heat flux, which can enhance both heat transfer and deterioration. The influence of the heat flux can be used to measure the fuel temperature inside the channel.

Computational Investigation of Micro-channel Heat Sink with Rectangular Shape Obstacles

Nowadays a lot of interest is given to the geometrical modification of heat sink systems to cool down the electronic components. To improve the performance index of the heat sinks, the use of geometrical features with different shapes and at different locations on the surface can be a valuable approach. In this paper, the effect of rectangular shape obstacles on the micro channel heat sink (MCHS) performance is studied. Due to surface features, vortex is developed which helps to increase the heat transfer rate. Numerical modeling software Comsol Multiphysics with heat transfer in fluid physics is used to investigate the characteristics of a micro-channel heat sink. The numerical result shows that the heat transfer rate can be improved through an appropriate arrangement of rectangular shape obstacles, on the heat sink. Numerical analysis and the comparison is carried out for micro-channel heat sink with and without obstacles. In this paper, various parameters like temperature rise, cell Peclet number and Mean effective thermal conductivity are studied.

Efficiency gain and bidirectional operation of quantum engines with decoupled internal levels.

We present a mechanism for efficiency increase in quantum heat engines containing internal energy levels that do not couple to the external work sink. The gain is achieved by using these levels to channel heat in a direction opposite to the one dictated by the second law. No quantum coherence, quantum correlations or ergotropy are required. A similar mechanism allows the engine to run "in reverse" and still produce useful work. We illustrate these ideas using a simple quantum Otto cycle in a coupled-spin system. We find this engine also exhibits other counterintuitive phenomenology. For example, its efficiency may increase as the temperature difference between the heat baths decreases. Conversely, it may cease to operate if the hotter bath becomes too hot or the colder bath too cold.

Details on the Hydrothermal Characteristics within a Solar-Channel Heat-Exchanger Provided with Staggered T-Shaped Baffles

Details on the hydrothermal characteristics of turbulent flows in a solar channel heat exchanger (CHE) are highlighted. The device has transverse T-shaped vortex generators (VGs). Two staggered VGs (baffles) are inserted on the lower and upper walls of the CHE. The working fluid is Newtonian and incompressible, with constant physical properties. The ANSYS Fluent 17.0 is utilized in this survey. The second-order upwind and QUICK schemes were utilized to perform the discretization of pressure and convective terms, respectively. The SIMPLE algorithm was employed to achieve the speed-pressure coupling. The residual target 10−9 was selected as a convergence criterion. The effects of the T-VGs’ geometrical shape and Reynolds numbers were inspected. At the baffle level, the wall effect was augmented due to the reduction of the passage area of flows, which is estimated here to be 55%, resulting thus in a considerable resistance to the movement of fluid particles. The thermal distribution is highly dependent on the flow structures within the CHE. Since the fluid agitation yields an enhanced mixing, it allows thus an excellent heat transfer. The most considerable rates of thermal transfer were obtained with high Re, which resulted from the intensified mixing of fluid particles through the formation of recirculation cells and the interaction with the walls of the T-VGs and the CHE. The T-baffles with intense flow rates yielded negative turbulent speeds and intensify the fluid agitation, which improves the thermal exchange rates.

Numerical analysis of flow and heat characteristic around micro-ribbed tube in heat exchanger system

In order to improve the energy utilization rate, this paper analyzes the flow around the cylinder in the tube-row channel heat exchanger through numerical simulation. Effects of micro-rib structure types (vertical rib and ring rib), number of ribs (N = 0, 4, 6, 8), height of ribs (H = 2, 3, 4 mm), pitch diameter ratio (ST/D = 1.5, 2, 2.5), mass fractions (ω = 0–0.5%) and Reynolds numbers (Re = 514–1205) on the flow and heat transfer characteristics of tube-row channel heat exchangers are analyzed. Results showed that TiO2-H2O nanofluids can improve the comprehensive performance of heat exchanger, and when the mass fraction is 0.4%, the tube-row channel heat exchanger has the best performance. For heat exchangers with different types and structural parameters, the adjustment of the number of ribs, the height of ribs, the ratio of transverse pitch diameter and the use of nanofluids can increase Nusselt numbers by 53.63%, 57.25%, 52.38% and 12.93% at maximum, respectively.

Effect of rib diameter on flow boiling heat transfer with staggered rib arrays in a heat sink

With the integration development of electronic equipment, a more compact and higher efficiency heat dissipation system is required to meet its cooling demands. In this work, the design of ribbed channel heat sink in a limited volume to maximize its heat transfer capacity is analyzed. Two-phase flow boiling performance of rib diameters ranging from D = 1–10 mm is characterized using the Mixture multiphase model. The working fluid is coolant Novec649 and its mass flux is 1527 kg m−2 s−1. The heat fluxes imposed on the channel wall range from 50 to 300 kW m−2. The results show that the average heat transfer coefficient increases and the wall temperature decreases with the increase of the rib diameter, respectively. The heat transfer performance factor increases with the decrease of heat fluxes, and first decreases with the increase of the rib diameters, then keeps constant. The comprehensive heat transfer performance factor is defined by temperature uniformity. It appears two maximum values which are located at D = 5 and 10 mm, one minimum value at D = 7.14 mm. The best comprehensive heat transfer performance of the heat sink is at D = 10 mm.

Impact of Ceramic Matrix Composite Topology on Friction Factor and Heat Transfer

AbstractCeramic matrix composites (CMCs) are of interest for hot section components of gas turbine engines due to their low weight and favorable thermal properties. To implement this advanced composite in a gas turbine engine, characterizing the influence of CMC’s surface topology on heat transfer and cooling performance is critical. However, very few published studies have reported the flow and heat transfer effects caused by this unique surface topology. This study is an experimental and computational investigation to evaluate the effect of weave orientations, relevant to CMC surfaces, on the resulting pressure loss and convective heat transfer within an internal channel. The weave pattern was additively manufactured as the walls of a scaled-up coupon containing a single channel. For each of the three weave orientations, bulk pressure losses and convective heat transfer coefficients were measured over a range of Reynolds numbers. Scaling the pressure losses in terms of a friction factor and convective heat transfer coefficients in terms of a Nusselt number showed the importance of choosing the appropriate definition of the hydraulic diameter, which was particularly important for the friction factor. A coupon having one wall with the weave surface increased pressure loss and heat transfer compared to a smooth wall with the largest increases occurring when the CMC weave strands were perpendicular to the flow. Friction factor augmentations were much higher than heat transfer augmentations. When adding the weave to a second channel wall, pressure loss and heat transfer were further increased. Orienting the CMC strands perpendicular to the flow consistently showed the largest augmentations in heat transfer over a smooth channel, but at a much higher pressure loss penalty than that seen with the CMC strands parallel to the flow.

Impact of velocities and geometry on flow components and heat transfer in plate heat exchangers

• Influence of velocity on flow is investigated experimentally and numerically. • The flow is mainly determined by substreams velocity and angle β . • Higher velocities promote furrow component and at fixed Re entail lower f and Nu. • Redefined Re , Nu and f shall reflect substreams flow path and velocity. • Numerical results are best validated in terms of heat flux and pressure drop. This study aims at better understanding of the complex flow phenomena in plate heat exchangers. This is identified in the literature to be crucial for improvement of numerical models and generalized heat transfer and pressure drop correlations with respect to their applicability in heat exchanger optimization. 3D numerical simulations of the flow in corrugated channels of plate heat exchangers with different corrugation angles ( β 28° and 65°) were performed in order to investigate in detail complex flow patterns and their influence on the heat transfer and pressure drop at laminar, transient and turbulent flow conditions. The study is supported by the visualization and thermal–hydraulic tests ( Re = 0.1–6037) carried out on the channels with corresponding chevron angles, using fluids of different viscosity to explore effects of different velocities at fixed Re numbers. Consequent alternation of flow patterns is believed to cause recorded differences in heat transfer and pressure drop data. Results from the simulations are analysed in different sections of the channels in light of the flow substreams, velocity distribution, wall shear stress and wall heat flux distribution. They confirm assumptions from the present visualization tests on the longitudinal and furrow substreams interactions and characteristics, as well as on the thermally and hydrodynamically developing flow conditions within a basic cell. A suitability of comparing simulations and experimental data in terms of Fanning friction factor and Nusselt number is discussed relative to the definitions of characteristic length, hydraulic diameter and mean velocity at different flow patterns. The computed heat transfer and pressure drop is in a relatively good agreement with the performed tests and literature data. The presented numerical-experimental analysis outlines importance of the substreams velocity on their interactions and flow components size, which in turn determines PHE channel heat transfer and pressure drop characteristics. The present findings may serve for improvement of CFD modelling, generalized correlations and definition of Re , Nu and f to allow for comparison of heat transfer and pressure drop data at different geometries and flow conditions.

Condensation heat transfer performance in multi-fluid compact heat exchangers with wavy and strip fins

The effect of different geometrical parameters of wavy and strip fins in a three-fluid plate-fin compact heat exchanger on the condensation heat transfer in the middle heat exchanger has been experimentally investigated. The three-fluid heat exchanger consisted of three heat exchangers with strip and wavy fins. In the examined system, the hot and cold fluids flowed through the adjacent heat exchangers (strip channels), while the middle heat exchanger (wavy channels) was occupied with a single-component two-phase flow (condensed vapor). The Nusselt number, the friction factor, the heat transfer surface contribution between the middle heat exchanger and each of the adjacent heat exchangers, and the thermal performance factor were calculated using the measured data at the inlets and outlets of the heat exchangers, and by using the thermal balance equations between the fluids passing through the heat exchangers. In the examined heat exchanger, the hot and cold flows had a cross-direction with the two-phase fluid flow. The two-phase Reynolds number, the vapor quality, and the heat transfer surface contribution of the middle heat exchanger with the adjacent heat exchangers were 50–4800, 0.1–0.67, and 0.19–0.7, respectively. The results show that the two-phase flow pattern in the wavy channels of the middle heat exchanger is stratified and wavy. According to the findings, a higher thermal performance factor is observed when the ratio of the wavy channel heat transfer surface to the heat transfer surface of the cold or hot flows is closer to 1. Also, with higher wave amplitude to wavelength ratios, lower pitch to height ratio of the fins, the lower wavelength to the fin length ratios, and lower fin pitch to wave amplitude ratios are obtained a higher thermal performance factor.

INFLUENCE THE RIBS IN NEW MODEL OF CHANNEL PATH ON HYDRO-THERMAL PERFORMANCE IN SERPENTINE MINI- CHANNEL HEAT SINK

The aim of the present study is to enhance of heat transfer in mini channel heat sink. To reach the target of this study a new models of mini channel heat sink is proposed the traditional model, model (A) and model (B). Both model (A, B) has straight serpentine mini channel heat sink with inlet at the center, the difference between them is that model(B) has ribs within channel, the diameter of ribs is 1 mm with distance between ribs 5mm. A 3D (ANSYS Fluent program) 2019R3 is used to obtain the numerical outcomes. A good agreement is found when compared the experimental outcomes from the literature review and the numerical results of the traditional model in the current study. Furthermore, the findings demonstrate that the formation of flow has a great impact on heat transfer, Though the model with serpentine straight channel improve the pressure drop, thermal resistance, Nusselt number and the distribution of temperature in the base of heat sink. The overall performance factor (OPF) for new models under study are superior than conventional model. In addition, the average OPF for model (A) is [1.43] and for model (B) is [1.33] as compared with traditional model. As well, model (A) is superior as compared with model (B) by 6.99 % due to the effectual uniformity in the base of heat sink and efficient OPF.

Enhanced heat transfer characteristics of water based hybrid nanofluids with graphene nanoplatelets and multi walled carbon nanotubes

A detailed experimental work on the convective heat transfer characteristics of water based hybrid nanofluids with Graphene Nanoplatelets and Multi Walled Carbon Nanotubes in the ratio 1:1 has been performed. Volume concentrations viz., 0.01%, 0.05%, 0.1%, 0.15% and 0.2% were considered. The hybrid nanofluid was passed through the micro channel heat sink which was electrically heated to simulate the heat load. The heat load was varied from 50W to 200 W while the flow rate of nanofluid was varied from 5 g/s to 30 g/s. The thermal conductivity of the 0.2 vol% nanofluid was 25% higher than the base fluid while the convective heat transfer coefficient was enhanced by 85% leading to a reduction of heat sink temperature by 12 °C. Variations of friction factor during nanofluid flow were nearly constant with respect to Reynolds number and volume concentration of the nanomaterials. This shows the absence of any penalty in the pressure drop with the addition of nanomaterials. The results showed the hybrid nanofluid (Graphene Nanoplatelets /Multi Walled Carbon Nanotubes-water) as an effective coolant for the replacement of the conventional coolant for electronic cooling applications.