Variation Of Local Heat Transfer Research Articles

Dual-fluid topology optimization of printed-circuit heat exchanger with low-pumping-power design

This study proposes a novel design for a printed-circuit heat exchanger (PCHE) using a dual-fluid topology optimization method. To account for the three-dimensional (3D) heat transfer phenomena using the two-dimensional (2D) computational domains, three distinct physical domains are defined: two design domains with density fields of hot and cold fluids, and a thermal conduction domain placed between both fluid fields. To account for the local heat transfer variation within a design domain, the local heat transfer coefficient is expressed using the density fields for the hot- and cold-fluid domains. Here, the design geometry had pillar-shaped fixed density values considering the metal 3D printing. Each flow region had a constant flow rate as the boundary condition with the constraint of the maximum pressure drop between the inlet and outlet. The total amount of the heat transferred between the hot and cold domains was maximized as an objective function. For the validation, the thermal performance of the topology-optimized PCHE was compared to that of a conventional PCHE. The topology-optimized PCHE showed a 66% higher heat transfer rate compared to the conventional PCHE under identical pumping-power conditions.

MHD mixed convection of nanofluid due to an inner rotating cylinder in a 3D enclosure with a phase change material

PurposeThe purpose of this study is to numerically examine the mixed convection of CuO-water nanofluid due to a rotating inner hot circular cylinder in a 3D cubic enclosure with phase change material (PCM) attached to its vertical surface. Heat transfer and fluid flow characteristics were examined for various values of pertinent parameters.Design/methodology/approachFinite element method was used in the numerical simulation. Influence of various pertinent parameters such as Rayleigh number (between 10$^5$ and 10$^6$), Hartmann number (between 0 and 100), angular rotational speed of the cylinder (between −50 and 50), solid nanoparticle volume fraction (between 0 and 0.04) and PCM parameters (height-between 0.2H and 0.8H, thermal conductivity ratio- between 0.1 and 10) on the convective heat transfer characteristics are numerically studied.FindingsIt was observed that local heat transfer variations along the hot surface differ significantly for the cases with and without magnetic field where three distinct hot spots of peak Nusselt number are established when magnetic field is imposed. The average Nusselt number enhancement with the nanofluid at the highest particle volume fraction is 52.85 per cent at Hartmann number of 100, whereas its value is 39.76 per cent for the case in the absence of magnetic field. When the inner cylinder rotates, flow and thermal fields are affected within the cavity. The local heat transfer variations spread over the hot surface with cylinder rotation and 16.43 per cent of reduction in the average heat transfer is obtained with counter-clockwise rotation at 100 rad/sec. An enhancement in the PCM height and a reduction in the thermal conductivity of the PCM result in average heat transfer deterioration for the 3D cavity. The amount of the reduction is 43 per cent when the PCM height is increased from 0.2H to 0.8H, whereas 19.10 per cent enhancement in the heat transfer is achieved when thermal conductivity ratio (PCM) to the base fluid is increased from 0.1 to 10.Originality/valueSuch configurations can be designed for convection control, and in our case, various methods are available. Some of the investigated methods can be used in applications where magnetic field already exists. Convection control study in 3D cavity gives more realistic results as compared to 2D configurations, and results of the current investigation may be used for the design, optimization and flow control of many thermal applications involving magnetic field effects.

Role of vane configuration on the heat dissipation performance of ventilated brake discs

This study reports on the effects of vane outlet angle on thermo-fluidic characteristics of curved vane brake discs via validated numerical models. Three vane outlet angles (45°, 90° and 135°) are considered. For a given rotational speed within the operating range of 200–1000 rpm, results show that increasing the outlet angle from 45° to 135° enhances the overall cooling performance by up to 16%. It is found that the increasing outlet angle enhances circumferential pressure difference, which intensifies secondary flow perpendicular to the mainstream and hence evidently enhances local heat transfer on inner disc surface. Further, the increase of outlet angle results in stronger flow impingement on pressure sides of the vanes and hence enhances local heat transfer. However, severe flow separation endows the brake disc having the outlet angle of 90° with the poorest heat transfer on suction sides of the vanes while those for the others are similar. In addition, local heat transfer variation on rubbing surface, inner rim surface and outer rim surface due to the variation of outlet angle has negligible effect on overall cooling performance. Comprehensive heat transfer enhancement mechanisms clarified in the present study are helpful to motivate further improved designs.

Three‐dimensional thermal investigations at board level in a reflow oven using thermal‐coupling method

PurposeThe purpose of this paper is to develop thermal modelling to investigate the thermal response of sample boards (at board level) during the preheating stage of the reflow process and to validate with experimental measurements.Design/methodology/approachA thermal‐coupling method that adopted the Multi‐physics Code Coupling Interface (MpCCI) was utilized. A forced‐convection reflow oven was modelled using computational fluid dynamic software (FLUENT 6.3.26), whereas structural heating at the board level was conducted using finite‐element method software (ABAQUS 6.9).FindingsThe simulation showed a complex flow pattern having characteristics of a free‐jet region, stagnation‐flow region, wall jet‐region, recirculation region and vortices. A sharp maximum heat‐transfer coefficient was detected in the stagnation region of the jet, resulting in a spatial variation of local heat transfer on a thermal profile board (TPB). This coefficient affected the temperature distribution in the TPB with different specific heat capacitances and thermal conductivity of the structure. The simulation results were in good agreement with the experimental data and analytical model. The cold region and temperature uniformity (ΔT) increased with increasing complexity of the TPB. The cold region can occur in two possible locations in the TPB. Both occurrences can be related to the flow field of the reflow oven. ΔT of the TPB decreased when the conveyor speed (v) was reduced. A suitable conveyor speed (1.0 cm/s) was determined to maintain ΔT below 10°C, which prevented the thermally critical package from overheating.Practical implicationsThe paper provies a methodology for designing a thermal profile for reflow soldering production.Originality/valueThe findings provide fundamental guidelines to the thermal‐coupling method at the board and package levels, very useful for accurate control of ΔT at the board and package levels, one of the major requirements in achieving a high degree of reliability for electronic assemblies.

The Simulation Study of Air Cooler in an Integrated Coal Gasification Combined Cycle System

A shell-and-tube heat exchanger which is widely used for the heat recovery of air in a coal gasification system is investigated. FLUENT6.3 software is used and the RNG k−ε turbulence model is adopted for modeling turbulent flow. The porosity rate, the distribution of the resistance and the distribution of the heat source were introduced to FLUENT by coupling the user defined function. The variation of local heat transfer is studied under the effects of the baffles arrangement. The result shows that the porous media model can be applied in shell side of the air cooler in IGCC and the reasonable arrangement of the baffles can decrease the pressure drop with little influence on the heat transfer.

Analysis of Heat Exchangers Used in the Integrated Gasification Combined Cycle Process

A shell-and-tube heat exchanger that is widely used for the heat recovery of syngas in a coal gasification system is investigated. FLUENT6.1 software was used, and the renormalization group k- turbulence model was adopted for modeling turbulent flow. The porosity rate, the distribution of the resistance, and the distribution of the heat source were introduced to FLUENT by coupling the user-defined function. The variation of local heat transfer was studied under the effects of the inlet velocity and the arrangement of the baffles. The result shows that a porous media model can be applied in the shell side of a syngas heat exchanger in the integrated gasification combined cycle. The pressure gradient is not directly related to the inletmass flow rate. The heat transfer of the second half of the shell side becomes strongerwith the increase of the inletmass flow rate.Widening the space between the baffles hasmore effect in decreasing the resistance and heat transfer than shortening the height of the baffles.

Investigations of axial and radial variations of heat transfer coefficient in bubbling fluidized bed with fast response probe

A fast response probe used in this study allowed measurements of local heat transfer variations with time in a gas fluidized bed. These variations have been related to local hydrodynamic conditions in the fluidized bed. Measurements have been carried out in a Plexiglas column of inside diameter 0.15m in the fluidized bed section and height of about 2m. The bed consisted of 150μm sand particles of static bed height 0.45m. The gas velocity was varied up to 0.4m/s. The bed hydrodynamics could be mapped by the radial and axial variations in heat transfer coefficients captured by the probe. The limitations and applicability of literature correlations and models for predictions of heat transfer coefficient in gas fluidized bed are pointed out. A correlation to predict heat transfer coefficients at a given radial location is proposed.

Optimization of high-pressure shell-and-tube heat exchanger for syngas cooling in an IGCC

This work investigates the flow field and the heat transfer characteristics of a shell-and-tube heat exchanger for the cooling of syngas. Finite volume method based on FLUENT software was used and the RNG k– ε turbulence model was adopted for modeling turbulent flow. The porosity rate, the distribution of the resistance and the distribution of the heat source are introduced to FLUENT by coupling the user defined function. The pressure drop, the temperature distribution and the variation of local heat transfer are studied under the effects of the syngas components and the operating pressure, and the effect of the arrangement of the baffles on the heat transfer is studied. The results show that higher operation pressure can improve the heat transfer, however brings bigger pressure drop. The components of the syngas significantly affect the pressure drop and the heat transfer. The arrangement of the baffles influences the fluid flow.

CFD investigation of the flow and heat transfer characteristics in a tangential inlet cyclone

This work presents a computational fluid dynamics (CFD) calculation to investigate the flow field and the heat transfer characteristics in a tangential inlet cyclone which is mainly used for the separation of the dens phase of a two phase flow. Governing equations for the steady turbulent 3D flow were solved numerically under certain boundary conditions covering an inlet velocity range of 3 to 30 m/s. Finite volume based Fluent software was used and the RNG k−ɛ turbulence model was adopted for the modeling highly swirling turbulent flow. Good agreement was found between computed pressure drop and experimental data available in the literature. The structure of the vortices and variation of local heat transfer were studied under the effects of inlet velocity.

Two-phase heat transfer coefficients of three HFC refrigerants inside a horizontal smooth tube, part I: evaporation

This paper presents experimental heat transfer results obtained during the evaporation of Isceon 59, R407C and R404A in a horizontal tube. The results have been compared with existing correlations which characterise the evaporative heat transfer coefficient to assess the validity of these models for refrigerant mixtures. The results compared well with the (Gungor K.E., Winterton, R.H.S. Simplified general correlation for saturated flow boiling and comparisons of correlations with data. The Canadian Journal of Chemical Engineering, Chemical Engineering Research & Design 1987;65(2):148–156 and Shah M.M. Chart correlation for saturated boiling heat transfer: equations and further study. ASHRAE Transactions 1982;88(1):185–196) correlations over a 0.2 to 0.8 vapour quality range. These correlations, however, did not accurately predict the results obtained during the dry-out at high vapour quality. A model, developed by (Kattan N., Thome J.R., Favrat D., 1998. Flow boiling in horizontal tubes: part 1 — development of a diabatic two-phase flow pattern map. Journal of heat transfer, transactions of ASME, Vol. 120, pp. 140-147; Kattan N., Thome J.R., Favrat D., 1998. Flow boiling in horizontal tubes: part 2 — new heat transfer data for five refrigerants. Journal of Heat Transfer, Transactions of ASME, Vol. 120, pp. 148–155; Kattan N., Thome J.R., Favrat D., 1998. Flow boiling in horizontal tubes: part 3–development of a new heat transfer model based on flow pattern. Journal of Heat Transfer, Transactions of ASME, Vol. 120, pp. 156–165), was found to express the local variations in heat transfer during the whole evaporation process. This model was modified to fit the three new refrigerants. The modified Kattan model offers a good prediction of the heat transfer results, with a standard deviation of 6.1%.

Local heat transfer measurements of an orthogonally rotating square duct with angled rib turbulators

Abstract The effect of angled rib turbulators on the heat transfer of an orthogonally rotating square duct were experimentally investigated. The Reynolds and rotation numbers were varied in 10,000–20,000 and 0–0.08, respectively. The leading and trailing walls were rib-roughened with angles of 90° and 60°. The results showed that the rotation induced a peripheral variation of the heat transfer and the higher heat transfer was seen using the 60° angled rib; judging from the local heat transfer variation, the skewed secondary flow induced by the angled rib seemed to result in additional heat transfer enhancement.

Local Heat Transfer Measurements of an Orthogonally Rotating Square Duct with Angled Rib Turbulators.

The effects of angled rib turbulators on the heat transfer of an orthogonally rotating square straight duct were experimentally investigated. The Reynolds number based on mean velocity and hydraulic diameter was varied from 10000 to 20000 and the Rotation number from 0 to 0.08. The leading and trailing walls were rib-roughened with angle-of-attack of 90 or 60 degree. The rib height-to-hydraulic diameter ratio was 0.1 and the rib pitch-to-hydraulic diameter ratio was 1. The results showed that the angled rib case gave higher heat transfer than the results of a 90 deg rib duct and a smooth duct. Judging from the local heat transfer variation, the skewed secondary flow induced by the angled rib seemed to give additional contribution to the heat transfer enhancement. The rotation made the heat transfer of the pressure side higher and that of the suction side lower, although the overall Nusselt number stayed almost constant within the present experimental range.

Modeling paint and adhesive cure in automotive applications

In the development of thermoset coating and adhesive formulations, optimum cure conditions are defined based on measurements of physical properties, chemical resistance, and durability. The range of cure conditions over which a thermoset material exhibits acceptable performance constitutes a cure window for the material, and can be used to define acceptable limits of variability for production processes. There is variability intrinsic to the stoving of complex articles of manufacture: local variations in heat transfer and thermal mass result in substantial variations in temperature history. By applying chemical understanding of crosslinking processes and basic chemical engineering principles, cure strategies can be evaluated using a combination of heat transfer and chemical kinetic models. Examples drawn from research on automotive paints and adhesives are given.

Local Convective Behavior and Fin Efficiency in Shallow Banks of In-Line and Staggered, Annularly Finned Tubes

Local mass transfer data for high-profile fins in the second row of in-line and staggered, circular-finned tubes are presented for Reynolds numbers from 5000 to 28,000 based on hydraulic diameter and velocity at the minimum flow area. The data, obtained using an optical adaptation of the naphthalene sublimation technique, show that local variations in heat transfer do not cause significant fin efficiency deviations from the analytical solution of Gardner (contrary to earlier reports). Average heat transfer and pressure drop data indicate that the thermal performance of the in-line arrangement is comparable to the staggered configuration.

Microstructural defects in high productivity twin-roll casting of aluminium

Extensive casting trials have been conducted on an experimental twin roll caster, and the microstructures of a range of alloys have been characterised. Two defects, surface bleeding and hot tearing, are observed after casting at high speed. Surface bleeding is the result of the squeezing of solute-rich liquid on to the surface of the solidifying sheet by the pressure of the rolls. Hot tearing is the result of local variations in heat transfer between the strip and the roll, causing differing amounts of rolling deformation in adjacent parts of the strip.

The effect of inclination on local heat transfer under an axisymmetric, free liquid jet

The effect of jet inclination on the local heat transfer under an obliquely impinging, round, free liquid jet striking a constant heat flux surface is investigated experimentally. The problem parameters investigated are jet Reynolds number in the range 6600–52000, and jet inclination, ranging from 40 to 90 deg, measured from the horizontal. Experiments are carried out for nozzle sizes, d = 4.6 and 9.3 mm. It is found that the point of maximum heat transfer along the x-axis (the line of intersection of the jet inclination plane with the impingement surface) is shifted upstream (with respect to the jet flow) as a function of jet inclination with a maximum observed shift of 0.5 nozzle diameters. In addition, it is found that the shape of local Nusselt number profiles along the x-axis changed as the jet is inclined. One of these changes is a sharpening of the peak in the profile at the point of maximum heat transfer. Another change is an increasing asymmetry around the point of maximum heat transfer with the upstream side of the profile dropping off more rapidly than the downstream side. Some measurements are taken to characterize the full two-dimensional heat transfer distribution under the inclined jet. Correlations are developed for the radial variation of local heat transfer in the plane of nozzle inclination.

Fluid Flow and Heat Transfer in a Parallel Plate Duct Containing a Cylinder

As an augmentative means of forced convective heat transfer, a cylinder was inserted in a parallel plate duct and the resultant local heat transfer coeffidient on the duct wall at R/L=0.4∼0.8(R : radius of cylinder, L : a half width of duct) was measured. It was clarified how heat transfer corresponded to the fluid flow behind the cylinder and near the wall and some consideration on the mechanism of augmentation in heat transfer was made. Experimental correlation has been obtained which makes it possible to calculate the local heat transfer variation in all the flow regions both upstfeam and downstream of the maximum heat transfer position for various R/L.

Heat Transfer by Dual Jet Discharged into a Rectangular Duct

The results are presented of an experimental study on the flow and heat transfer in a two-dimensional parallel duct in the presence of a recirculating zone. The formation process of recirculating flow changes at about a/L0.6 (a =L-s, L: a half width of duct, s: nozzle width), depending on whether Karman vortex is shed or not. An experimental correlation has been obtained which makes it possible to calculate the local heat transfer variation in all the flow regions downstream of the maximum heat transfer position for various a/L. Some consideration on the maximum heat transfer mechanism is made in connection with the behavior of recirculating flow.

Fine Wire Thermocouple Measurements of Fluctuating Temperature

Abstract A technique for the measurement of the statistics of the temperature fluctuation, notably probability density function and power spectrum, in turbulent diffusion flames which employs fine wire thermocouples, compensated for the effects of thermal inertia, is assessed. Measurements made in an open turbulent methane diffusion flame are reported and compared, from the standpoint of identifying the interaction between measurement technique and turbulent flame structure, with simple model predictions. The flame-intermittent zone is seen to be characterised by large fluctuations in temperature and velocity which lead to significant local variations in heat transfer to the thermocouple and hence in its time constant. Attention is therefore focussed on an assessment of the impact on measured temperature statistics of both the level of compensation and the manner it is effected. Techniques involving electrical networks and computational procedures are discussed.

The Evaluation Of A New Model To Simulate Heat Transfer Enhancement In Ribbed Channels

The improvement of overall performance in power to mass ratio of modern gas turbines can greatly be contributed to the effective cooling of the turbine blades. Optimization of coolant performance is essential as the coolant flow introduces losses which need to be minimized. This paper introduces a new model to simulate heat transfer in ribbed channels. In the ribbed sections large variations in turbulence levels occure and the secondary flows associated with these ribs are responsible for significant local heat transfer variations. Furthermore, the ribs produce a complex flow structure so that there is continuing need for a better understanding of the flow physics in ribbed ducts. A finite difference simulation of the fluid flow and temperature distribution in the inlet section of the cooling channel of a typically cooled turbine blade is used to evaluate the new model. Various configurations of rib turbulators in the blade passages were simulated. In the case of the 45° angled ribs, there is a strong sideways secondary flow component which produces both increased overall heat transfer as well as asymmetrical local heat transfer enhancement. The results show the flow structure and heat transfer enhancement and correlate local heat transfer variations to secondary flow details. In general the predicted heat transfer rates compare well with measured data for 90° ribs, but further work needs to be done to accurately model angled ribs. NOMENCLATURE