Increment In Heat Transfer Coefficient Research Articles

Experimental study on flow boiling characteristics of R-1233zd(E) of counter-flow interconnected minichannel heat sink

Two-phase mini/micro-channel heat sink is one of the most effective solutions to the heat dissipation of high-power electronic devices in a narrow space. Flow boiling instability and wall temperature nonuniformity are however the two notorious problems of two-phase mini/micro-channel heat sink that have hindered their practical applications to a large degree. This study proposed a counter-flow interconnected minichannel (CFIM) to improve the flow boiling stability and the temperature uniformity of a mechanically machined copper minichannel heat sink. Firstly, a comprehensive comparative study of CFIM and co-current minichannel (CCM) was carried out concerning two-phase flow patterns, boiling heat transfer characteristics, and wall temperature uniformity. Secondly, the effects of mass flux, subcooling and saturation temperature on the boiling heat transfer performance of the two kinds of heat sinks were further investigated. Experiments were carried out at a saturated temperature of 40 ∼ 50°C, mass flux of 291∼ 618 kg/(m2·s), and subcooling of 15 ∼ 30°C, using a new generation of environment-friendly coolant with a low boiling point, R1233zd(E). In particular, no backflow or partial dry-out is observed in the channel throughout all the tested conditions. Due to the mixing of fluid between adjacent channels with counter flow caused by interconnected slots in CFIM, low upstream vapor quality and high downstream vapor quality were avoided. Therefore, this counter-flow interconnected mechanism in the current design can be significantly enhanced in flow boiling stability and temperature uniformity, up to an effective heat flux of 230.4 W/cm2 with 51.2% increment of heat transfer coefficient, 56% increment of coefficient of performance (COP) and 48.5% reduction of pressure drop when compared with those of traditional CCM design. Overall, the research results have good guidance and reference for the application and research of two-phase minichannel heat sinks.

Performance of parabolic trough collector having rotating receiver: Effect of elliptical insert and hybrid nanofluid

This study considers three parabolic trough collector models with rotating receiver tubes and elliptical inserts of two orientations and one with a smooth receiver tube. Hybrid nanofluid is considered as a working medium by mixing copper oxide and aluminum oxide nanoparticles in water. Computational experiments are performed with a 1% concentration in terms of vol. of hybrid nanofluids by considering a mixture of copper oxide and aluminum oxide nanoparticles in water (aluminum oxide 25%- copper oxide 75%, aluminum oxide 50%- copper oxide 50%, and aluminum oxide 75%- copper oxide 25%). Rate of flow for the simulation is considered to vary from 0.016 to 0.033 kg/s. Again, this study focuses on utilizing rotating receiver tubes with a varying velocity of 0-15 rad/s to overcome the stratification of high temperature at the surface. Results show that enhanced values for the temperature at outlet, heat transfer coefficient, and thermal efficiency are noted for a model with elliptical inserts of major diameter 28 mm (longitudinal orientation) at 15 rad/s receivers speed for nanofluid-1 (aluminum oxide 75%- copper oxide 25%) at 0.033 kg/s. The highest increment in heat transfer coefficient, as well as thermal efficiency for the model with a 28 mm major diameter elliptical insert (longitudinal orientation), is 44.21% and 29.21% respectively. Highlights Use of a spinning receiver reduces the problem of uneven temperature distribution at receiver surface. Maximum 375 K outlet temperature is noted at 15 rad/s for elliptical insert of diameter 28 mm (longitudinal orientation) at 0.016 kg/s. The elliptical insert received the maximum thermal efficiency rating of 82.03%. An elliptical insert in receiver increases pump work by 18.13% than smooth receiver.

Effect of Taper and Perforation on Heat Transfer Coefficient of a Passive Horizontal Heat Sink

The present study focuses on the numerical investigation of a passive horizontal base heat sink with perforated fins. Fins of the rectangular and trapezoidal cross-section are considered for the study. A detailed parametric numerical study is carried out considering the thickness ratio range (0.33–1); 4–6 mm size of square perforation; 6, 12, and 18 number of perforation and Rayleigh number 592–3044. Significant effect of thickness ratio, size of square perforation, and the number of perforations is observed on the heat transfer coefficient. The heat transfer coefficient increases with the decrease in the thickness ratio and increases with the number of perforation and size of square perforation in trapezoidal cross-section fins. 11.7% of increment in heat transfer coefficient was observed in perforated heat sink when it is compared to the non-perforated heat sink. Mass of the passive heat sink decreases with an increase in the size of perforation, number of perforations, and decrease in the thickness ratio. Mass reduction of up to 58% is observed in perforated heat sinks. A significant improvement in the performance factor is observed for the present study parameters range of Rayleigh number, the thickness ratio, size of perforation, and the number of perforations.

Hybrid nanofluid flow over two different geometries with Cattaneo–Christov heat flux model and heat generation: A model with correlation coefficient and probable error

The authors scrutinize the steady, MHD flow of SiO2−MoS2/water hybrid nanofluid towards two different geometries i.e. a wedge and a cone. The Tiwari and Das model is implemented with a generalized–Fourier's model, popularized as Cattaneo-Christov heat flux model. Analysis of heat transfer also incorporates the effects of suction, heat generation and thermal radiation. To showcase the relationship between engineering quantities and pertinent parameters involved in the study, the correlation coefficient for heat transfer coefficient and the skin friction coefficient is computed followed by the computation of probable error and statistical declaration. Similarity transformations are utilized to remodel the constitutive laws of flow in non-dimensional form. Numerical computation of non-linear, coupled O.D.E.’s is performed with the support of the Runge-Kutta-Fehlberg scheme and shooting method. Graphical and tabular illustrations of computed results are provided to report the variation in flow properties with the fluctuation in physical parameters. In both cases, i.e. flow close to a wedge and a cone, the temperature of hybrid nanofluid enhances on intensifying the thermal radiation and experiences a decrement with thermal relaxation parameter and magnetic field. Rising values of the suction parameter, thermal relaxation parameter, and thermal radiation cause increment in heat transfer coefficient. Interestingly, it was spotted that the heat generation parameter has contrary effects on temperature distribution over the two geometries.

Effect of Magnus Force on Flow over Cylinder and Heat Transfer Investigation: A Computational Fluid Dynamics Approach

This computational study deals with the forced convection through horizontal cylinder which is performed in the CFD Lab, Department, Mechanical Engineering, GLA University, Mathura. The convective heat transfer coefficient is measured for two different cases stationary cylinder and rotating cylinder. The computational analysis was performed by using commercial CFD package ANSYS-Fluent. This analysis is done because we can’t get the pressure and velocity values in the full domain by using the governing equations. The contours of pressure, velocity and temperature are determined to capture the behaviour of flow throughout the region. Computationally determined values of local heat transfer coefficient are compared with the theoretical values obtained by using governing equations. The effect of the Magnus force is measured on the computational value in such way whether there is any increment in heat transfer coefficient or not. The nondimensional numbers as Nusselt number (Nu), Reynolds number (Re) and Prandtl number (Pr) are used to find the results of heat transfer. It is found that heat transfer is more efficient in case of rotating cylinder in comparison with stationary cylinder. The computationally determined results are found in well agreement with the analytical result.

Modeling of Newtonian and non-Newtonian-based coolants for deployment in industrial length-scale shell and tube heat exchanger

To evaluate the heat transfer performance (HTP) of hybrid nanofluids, numerical simulations are carried out in an industrial length single pass shell and tube heat exchanger. In shell, ISO VG 68 oil enters with [Formula: see text]C and with [Formula: see text]C, the coolant passes into the tube. CNT-[Formula: see text]/water and CNT-[Formula: see text]/sodium alginate (SA) are used as Newtonian and non-Newtonian hybrid nanofluid, respectively. The influence of base fluid and nanoparticles on thermal performance of heat exchanger is studied. The chosen nanoparticles are reliable to the industrial deployment. The current numerical procedure is validated with the earlier experimental results. Volume fraction of nanoparticles is optimized for an effective HTP of the heat exchanger. About 60% increment in heat transfer coefficient is observed when hybrid nanofluid is employed. By using Newtonian hybrid nanofluid, 50% improvement in Nusselt number is marked out. Effectiveness and heat transfer rate of heat exchanger are higher with the employment of Newtonian hybrid nanofluid. Results indicated that, even though Newtonian hybrid nanofluid shows higher thermal performance, non-Newtonian hybrid nanofluid is preferable for energy consumption point of view.

Enhancement of Nusselt number by using CNT and CuO Nanofluids in Heat exchangers

Experimental Studies of CNT and CuO water based Nanofluids in heat exchangers were performed by varying flow rate from 2 to 4 Litres Per Minute and nanoparticle concentration from 0.1 to 0.3 percent by volume. Double pipe type and Shell & tube type heat exchangers were used for this study. The increment in heat transfer coefficient for both heat exchangers were determined and compared. It was found that the increment in Nusselt number for both CNT and CuO Nanofluids was more in shell and tube type as compared with Double pipe type heat exchanger.

Heat Transfer Enhancement of Supercritical Nitrogen Flowing Downward in a Small Vertical Tube: Evaluation of System Parameter Effects

In this paper, the heat transfer enhancement (HTE) of supercritical nitrogen flowing downward in a vertical small tube (diameter 2 mm) is studied using the commercial software CFX of Ansys16.1, to provide theoretical guidance on the design of high-performance heat transfer systems. An effective numerical simulation method, which employs the SSG Reynolds stress model with enhanced wall treatment, is applied to study the heat transfer of supercritical nitrogen under typical working conditions. The objective is to evaluate the effect of the main parameters taking into account the buoyancy and flow acceleration effects. Simulation results are compared with results calculated from three well-known empirical correlations and the applicability of empirical correlation is discussed in detail. It is discovered that the Watts and Chou correlation accurately fits the simulation results of supercritical nitrogen and the Dittus-Boelter and Jackson correlations can only be used for high-pressure conditions. The HTE of supercritical nitrogen is closely related to the laminar sub-layer and buffer layer of a boundary layer. The buoyancy effect on the HTE should be considered at low mass flux conditions, and thermal acceleration can be completely ignored for the cases studied. The special HTE featured by the increment in heat transfer coefficient with increasing heat flux is discovered at low pressure, and simulation results proved that this HTE is caused by the combined actions of buoyancy as well as significant variations in specific heat and viscosity.

High performance heat sink with counter flow diverging microchannels

Effective thermal management of high-power electronics is urgently demanded for the further development of electric vehicles. Two-phase flow boiling in microchannels is well recognized as a prominent approach for high density heat dissipation. This study develops a novel high performance heat sink with counter flow diverging microchannels to further enhance the flow boiling performance. A unique and significant heat exchange between the neighboring channels is demonstrated to dramatically alter the flow pattern transition in the counter-flow microchannel manifold. In particular, as the flowing bubbles in the microchannels can shrink or even disappear before exiting, no back flow or partial dry-out is observed throughout all the tested conditions. Therefore, this additional heat transfer mechanism in the present design enables significant enhancement of temperature uniformity, up to a heat flux of 2677 kW·m−2 with 45.1% increment of heat transfer coefficient, 73.8% reduction of pressure drop and 123.1% increment of coefficient of performance when compared with those of traditional co-current flow design. Overall, with the excellent flow boiling performance, the present study offers a highly promising microchannel design for a variety of applications requiring high heat flux dissipation.

Heat transfer enhancement through periodic flow area variations in microchannels

In this study, annular microchannels with a microscale gap of 300 μm were implemented through the concentric superposition of two macro-sized cylinders. Flow area variations along the streamwise direction were created by introducing sinusoidal wave profiles on either the inner or outer wall of the annular gap while keeping the other wall flat. These variations introduced re-entrant effects along the flow direction. Numerical studies using the finite volume method were performed to elucidate the single-phase, steady-state thermal and hydrodynamic performances of the wavy channels, using water as the fluid medium, with an operating Reynolds number range of 800–2200. The predicted results were validated using the available measured data and classical correlations. This study demonstrated the viability of attaining enhanced heat transfer rates of up to 360% of the original straight channel through the inducement of flow area variations with single wavy-walled channels. Despite magnifications of the friction factors, the single wavy-walled channels attained a 120% increment in heat transfer coefficient when evaluated at the same pumping power. Overall, single-walled wavy passages were deemed suitable for heat exchanger designs demanding very high heat removal rates and efficiencies while the conventional serpentine channels were apt for moderately enhancing heat transfer while requiring low pumping power.

Experimental Investigation of Air–Water Mist Jet Impingement Cooling Over a Heated Cylinder

Experimental investigation on heat transfer mechanism of air–water mist jet impingement cooling on a heated cylinder is presented. The target cylinder was electrically heated and was maintained under the boiling temperature of water. Parametric studies were carried out for four different values of mist loading fractions, Reynolds numbers, and nozzle-to-surface spacings. Reynolds number, Rehyd, defined based on the hydraulic diameter, was varied from 8820 to 17,106; mist loading fraction, f ranges from 0.25% to 1.0%; and nozzle-to-surface spacing, H/d was varied from 30 to 60. The increment in the heat transfer coefficient with respect to air-jet impingement is presented along with variation in the heat transfer coefficient along the axial and circumferential direction. It is observed that the increase in mist loading greatly increases the heat transfer rate. Increment in the heat transfer coefficient at the stagnation point is found to be 185%, 234%, 272%, and 312% for mist loading fraction 0.25%, 0.50%, 0.75%, and 1.0%, respectively. Experimental study shows identical increment in stagnation point heat transfer coefficient with increasing Reynolds number, with lowest Reynolds number yielding highest increment. Stagnation point heat transfer coefficient increased 263%, 259%, 241%, and 241% as compared to air-jet impingement for Reynolds number 8820, 11,493, 14,166, and 17,106, respectively. The increment in the heat transfer coefficient is observed with a decrease in nozzle-to-surface spacing. Stagnation point heat transfer coefficient increased 282%, 248%, 239%, and 232% as compared to air-jet impingement for nozzle-to-surface spacing of 30, 40, 50, and 60, respectively, is obtained from the experimental analysis. Based on the experimental results, a correlation for stagnation point heat transfer coefficient increment is also proposed.

Heat transport and pressure drop characteristics of ethylene Glycol-based Nano fluid containing silver nanoparticles

In the current work, the heat transport characteristics of ethylene glycol with different volume fractions of silver nanoparticles dispersed to form a nanofluid that flows through an inner tube of the annular heat exchanger were investigated experimentally. The mass flow rate of nanofluid varied from 5 g/s to 25 g/s at 50°C inlet temperature of nanofluid on the heat transfer co-efficient were carried out under laminar flow conditions. The nanofluid was prepared with 0.05% and 0.1% volume percent of silver nanoparticles. The thermo-physical characteristics of pure ethylene glycol and silver/ethylene glycol nanofluid were measured experimentally. The heat transfer co-efficient was significantly increased with respect to Reynolds number and volume percent of silver nanoparticles. The experimental results show that the of convective heat transfer coefficient of the nanofluids increases up to 38% at a volume percent of 0.1% with those compared with basefluid at same Reynolds number. The enhanced thermal conductivity of nanofluid and the clustering of nanoparticles could be the probable reasons for the increment of heat transfer co-efficient. The pressure drop of the nanofluid increases as a function of Reynolds number and volume percent of nanoparticles. The increment in pressure drop is more predominant with respect to Reynolds number as compared to volume percent of nanoparticles.

Experimental investigation on Al2O3-R123 nanorefrigerant heat transfer performances in evaporator based on organic Rankine cycle

This paper presents an experimental investigation of variation tendency of heat transfer coefficient, log-mean temperature difference and differential pressure of pure R123 and 20 nm Al2O3-R123 nanorefrigerants with four various volume concentrations, 0.03%, 0.13%, 0.18%, 0.23% flowing inside the evaporator of organic Rankine cycle system under the conditions of various heat source temperatures and flow rates. Heat source temperatures are in the range of 50–90 °C at an interval of 10 °C, and heat source flow rates are 0.7 m3/h, 1.3 m3/h and 1.8 m3/h. Results show an increment of heat transfer coefficient along flowing direction for both pure R123 and four Al2O3-R123 nanorefrigerants with the increment of heat source temperature and flow rate, and that of four nanorefrigerants are higher than that of pure R123. There is no optimum value of heat transfer coefficient when operation condition is changed for the loading carrying capacity increasing with the increasing intensity of operation condition. Meanwhile, suspending nanoparticles and increasing heat source temperature can change the variation tendency of heat transfer coefficient along flowing direction except pure R123 and 0.18 vol% nanorefrigerant. In addition, 0.13 vol% as a whole is the optimum volume concentration for both log-mean temperature difference and differential pressure.

Experimental and Numerical Investigation of Forced Convection Heat Transfer in Heat Sink with Rectangular Plates at Varying Inclinations on Vertical Base

Heat exchange upgrade is a vital territory of research area. Utilization of reasonable systems can bring about noteworthy specialized points of interest coming about reserve funds of cost. Rectangular plates are viewed as best balance arrangement utilized for heat exchange improvement. This gives an enlargement strategy to heat exchange with beginning of limit layer and vortex development. To assess and look at the rate of heat exchange enhancement by rectangular plate fins with differing inclinations (0°–30°–60°), shifting Re and heat supply under forced convection are the principle destinations of this study. The study is done by fluctuating introductions of fins with various inclinations, input heat supply and Re under forced convection. The coefficient of heat transfer increments observed with the expansion in air speed for all the examined designs. The coefficient of the heat transfer is discovered higher at the edge of introduction of fins at 30° for inline arrangement and 0° for staggered arrangement. Looking at both the arrangements, it is discovered that the heat transfer coefficient in 0° fin staggered arrangement is about 17% higher than 30° inline arrangement and 76% higher than the vertical plate fin. For plate fin heat sink, boundary layer formation and growth results in decrease of the coefficient of heat transfer in forced convection. This issue is overcome by accommodating some rectangular fins on the plate fin. It brings about increment of heat transfer coefficient of the RPFHS under the states of trial factors. As indicated by past research, it is discovered that examination of the plate fin heat sink with various sorts of fins for horizontal orientation is done yet but this investigation expects to discover the upgrade of transfer coefficient of plate fin heat sink for its vertical position with rectangular plates at different inclinations under the shifting scopes of heat input supply, fin arrangements and Reynolds number (Re).

Heat transfer intensification using polyaniline based nanofluids: Preparation and application

The present work reports on the use of ultrasound assisted emulsion polymerization for the synthesis of polyaniline (PANI) nanoparticles. The study of heat transfer characteristics of PANI based nanofluids at different loading (0–1.2wt%) of PANI nanoparticles in water (reference fluid) was performed at fixed Reynolds number. The heat transfer coefficient enhancement was also studied at two different Reynolds numbers by keeping loading of PANI in the range of 0.2–1.2wt%. The heat transfer performance of PANI nanofluid was studied in a copper tube under constant heat flux conditions. The results showed that the heat transfer coefficient increases with an increase in the colloidal PANI concentration and Reynolds number. For 0.2wt% PANI nanofluids, 33% increment in heat transfer coefficient was observed while it was 63% for 1.2wt% addition of PANI nanoparticles.

806 機器の劣化・損傷評価への熱流体現象変化の取り込み

This paper describes the necessity of consideration aerothermodynamic phenomena change in evaluating deterioration and damage of hot parts in power generation apparatus. Concerning the turbine cooled rotor blade which are critical hot parts of gas turbine, the metal temperature of long term serviced blades became higher than that of short term serviced one. This change was caused by the increment of heat transfer coefficient on rotor blades because of the increase in blade surface roughness. The effect of aerothermodynamic phenomena change on the damage of the turbine cooled rotor blades is discussed.

Effects of Rolling on the Heat Transfer from Cango Oils of Tankers

A 1/50 scale model of side tanks of a tanker is used in this experiment to know effects of rolling on the heat transfer from shell walls. Results of the experiment are as follow.(1) The temperature of oil in the tank becomes uniforrn when rolling occurs.(2) The heat transfer coefficient increases almost linear according to the angle and frequency of rolling.(3) Fin effect of stiffeners is little, then their size and material affects little the heat transfer from stiffeners. They work mainly as mixers of oil in tanks.Empirical equations for increments in the heat transfer coefficient by rolling are given as, ΔNu=0.0160 Re0.942Pr1/3……for side shellΔNu=0.0155 Re0.910Pr1/3……for bottom shell where ΔNu=Δαl/λ and Re=2πbθl/ (Tν).(Δα : increment in heat transfer coefficient, l : stiffener space, λ : thermal conductivity, θ : rolling angle, b : half breadth of tank, T : rolling period, ν : kinematic viscosity.)