Forced Convection Heat Transfer Research Articles

A cost‐effective approach to generate accurate correlations via analyzing a minimum number of data points; Case study on the convection heat transfer problem

AbstractIt is very common in the heat transfer area to analyze and design heat equipment using the past available heat correlations. Basically, demanding higher‐accuracy correlations enforces the heat laboratories to test and collect larger banks of laboratory data. However, this conversely affects the laboratory cost. Therefore, it becomes challenging to create new approaches that let the correlation developers use smaller experimental datasets and provide correlations with sufficient accuracies. To surmount this challenge, the present work develops a new approach that benefits from the computational fluid dynamics method as a reliable and cheap tool and adequately enriches the original, insufficient dataset. Then, suitable enhanced correlations are developed using the new enriched experimental‐numerical‐based dataset. In parallel, the artificial neural network (ANN) is used to enrich the original insufficient dataset separately. Using this experimental‐ANN‐based dataset, it provides a totally ANN‐based correlation. It is shown that the results of enhanced correlations are as accurate as those of the ANN‐based correlation. However, the point is that the use of the present approach is about 100 times faster than using the ANN. The typical forced convection heat transfer through a pipe is examined here to show the capabilities of the current approach.

Key technologies of cooling and lubrication for closed hot extrusion automatic production line

For the problems that the stability problem that affects the fully automated production of precision extrusion of cylindrical parts, the cooling and lubrication of the die under high temperature and high pressure is the key technology that affects the production stability. During the hot extrusion process, the punch die has a short life under high temperature and high pressure, and cannot be Achieving continuous production and maintaining the life of the punch is the key technology to realize high-efficiency unmanned production of hot extrusion automation. First, the temperature rising and cooling formula for die especially for punch was obtained by theoretical calculation. The convective heat transfer coefficient on the surface of the internal cooling pipe was calculated and punch cooling device was made based on the forced convection heat transfer of cooling water. The changing law of the temperature field after punch extrusion was obtained by the infrared temperature measuring device and the test results were consistent with the theoretical calculation results.

Simulation for flow across heated square cylinders

AbstractThe thermal lattice Boltzmann method is used to examine forced convection heat transfer from six inline heated square cylinders for Re = 100 at 0.5 ≤ s/d ≤ 4.0, where s is the distance between the surfaces of two cylinders and d is the cylinder size. Such a heat transfer is transient in nature for which the present work reports heat transfer regimes such as synchronous, quasiperiodic and chaotic. For 0.5 ≤ s/d ≤ 1.5 the heat transfer is synchronous, for 1.5 ≤ s/d ≤ 3.0 it is quasiperiodic and for 3.0 ≤ s/d ≤ 4.0 it is chaotic in nature at Re = 100. These regimes are confirmed through cylinder Nusselt number signals, its power spectra, and heat wake interference. The appearance of heat transfer regimes for inline heated cylinders is similar to the appearance of flow regimes for inline unheated cylinders except for the fact that transition from synchronous to quasiperiodic regime occurs at s/d = 1.5 for heat transfer and at s/d = 1.1 for flow. The synchronous heat transfer regime is characterized by a single heat wake that envelopes all cylinders while quasiperiodic heat transfer regime is characterized by the formation of thermal blobs in the gap between cylinders. A chaotic heat transfer regime is characterized by the shedding of thermal blobs and interference of thermal blobs by downstream cylinders. Regardless of spacing, the average Nusselt number encountered by cylinders is smaller than that for the isolated cylinder. The novelty of the work is that transitions occurring in the flow of heat are considered for an understanding of heat flow from bluff bodies.

The Thermal Performance Analysis of an Al2O3-Water Nanofluid Flow in a Composite Microchannel

Partial filling of porous medium insert in a channel alleviates the tremendous pressure drop associated with a porous medium saturated channel, and enhances heat transfer at an optimum fraction of porous medium filling. This study pioneered an investigation into the viscous dissipative forced convective heat transfer in a parallel-plate channel, partially occupied with a porous medium at the core, under local thermal non-equilibrium condition. Solving the thermal energy equation along the Darcy-Brinkman equation, new exact temperature fields and Nusselt number are presented under symmetrical isoflux thermal boundary condition. Noteworthy is the heat flux bifurcation at the interface between the clear fluid and porous medium driven by viscous dissipation, in cases where the combined hydrodynamic resistance to fluid flow and thermal resistance to fluid conduction is considerable in low Darcy number porous medium insert. However, viscous dissipation does not affect the qualitative variation of the Nusselt number with the fraction of porous medium filling. By using Al2O3-Water nanofluid as the working fluid in a uniformly heated microchannel, partially filled with an optimum volume fraction of porous medium, the heat transfer coefficient improves as compared to utilizing water. The accompanied viscous dissipation however has a more adverse impact on the heat transfer coefficient of nanofluids with an increasing Reynolds number.

CFD analysis of a flat tube with semi‐circular fins using graphene nanofluid and to compare performance with square type of fins

AbstractCFD analysis on a flat tube with semi‐circular fins under laminar flow conditions was performed with graphene‐based nanofluids considering the nanofluids as incompressible. Different simulations were performed at four different concentrations of nanofluids (0.01%, 0.1%, 0.2%, and 0.4%) and at different volume flow rates (4, 6, 8, and 10 LPM) and at four different forced convective heat transfer coefficients at different wind velocities at 300 K (50, 100, 150, and 200 W/m2 K). It was observed that with an increase in the concentration of nanoparticles in nanofluids, the thermal conductivity of base fluid water was increased (at 353 K the nanofluid of 0.4% volume concentration, the thermal conductivity of nanofluid increased by 200% with respect to base fluid). Graphene‐based nanofluids have higher effectiveness than most nanofluids hence it is considered for the analysis, at 0.4% concentration of nanofluid the effectiveness observed was 36.84% at 4 LPM, and for water, the effectiveness was 28.22% under similar conditions. The effect of flow rate on temperature drop was significant. At 4 LPM and at 0.4% of nanofluid, an outlet coolant temperature of 333 K was observed whereas the water outlet temperature at 10 LPM is 346.13 K. The effect of forced convective air heat transfer coefficient was significantly high. At h = 50 W/m2 K the outlet temperature of 0.4% nanofluid at 4 LPM was 345.25 K and at h = 200 W/m2 K, the outlet coolant temperature was 333.47 K. A single tube of the radiator was considered for the analysis whereas the original radiator consists of 50 tubes due to problems of Ansys in meshing.

Hydrothermal study of nanofluid flow in channel by RBF method with exponential boundary conditions

Our study investigates the motion, temperature and volume fraction of a nanofluid that flows at a vertical channel exposed to natural and forced (mixed) convection heat transfer. The distribution function for temperature and volume fraction on the channel's wall is exponential. Nanofluid governing equations have been solved using the radial basis function method. Various parameters, including Grashof number, Brownian motion, thermophoresis parameter, plus distinct states for temperature and volume fraction functions for the channel's wall, have been probed. The results show that increasing the Grashof number by 50% will increase the fluid velocity and temperature by about 41% and 5.7% and decrease the volume fraction by 22%. In addition, it has been proved that doubling the Brownian motion parameter improves the fluid temperature and velocity by 67% and 66% and lowers the volume fraction by 53%. Moreover, increasing the thermophoresis parameter has a similar effect to the Brownian motion parameter. Finally, it has been seen that velocity, temperature and volume fraction functions strongly depend on temperature and volume fraction distribution functions on the channel's wall.

Jet array impingement boiling in compact space for high heat flux cooling

To achieve high heat flux cooling, a distributed confined jet array impingement boiling device was designed and tested by using HFE-7100 as working fluid. The experimental study on the heat transfer characteristics was conducted on smooth silicon surface and micro-pin-finned surfaces with mass flux ranging from 760 ∼ 3040 kg/m2·s under atmospheric pressure and an inlet subcooling of 40 K. The results indicated that with the increase of the jet velocity, nucleate boiling was suppressed, and the forced convection heat transfer was enhanced. The heat transfer was greatly intensified on micro-pin-finned surfaces with a maximum increase of the heat transfer coefficient of 220 % due to the increase in specific surface area and the number of nucleation sites. Moreover, the critical heat flux (CHF) can reach 280 W/cm2. The mechanism of CHF improvement was analyzed. The two-phase flow structure within the confinement space and capillary wicking effect of the micro-pin-finned surface are superimposed, resulting in two distinct CHF mechanisms. A new correlation with a mean absolute error of 3.5 % for predicting the heat transfer coefficient of the jet impingement boiling was proposed by considering the effect of micro-pin–fin structure on heat transfer.

Topology optimization of convective heat transfer by the lattice Boltzmann method

AbstractTopology optimization (TO) is a dependable approach to obtain innovative designs with improved performance. This study presents a TO method based on the adjoint lattice Boltzmann method (ALBM) and the level set method which is developed for both one‐way coupled and two‐way coupled convective heat transfer problems. The adjoint lattice Boltzmann model for fully coupled natural convection system is derived, and the coupled solution strategy is applied in the ALBM. The forward model is validated by the finite element simulation, while the adjoint model and the sensitivity expression is validated by a finite difference check, and the whole TO method is validated by a typical pure fluid flow optimization problem given in the literature. The validated TO method is then applied to enhance the heat transfer of forced convection in a two‐dimensional open chamber, and the Pareto frontier of the bi‐objective optimization is further presented and the effects of the blockages at the inlet and outlet on the overall performance are revealed. Finally, the two‐dimensional natural convection process in a closed cavity is optimized, in which the effective heat transfer coefficient can be increased to 3.96 ∼ 6.11 times of that without the optimized design when the Grashof number ranges from 1.7 to 4.2 × 105. Moreover, effects of Grashof number, porosity limitation and solid thermal diffusivity on the optimization results are analyzed in detail. Physically reasonable designs are obtained for both forced convection and natural convection systems under various parameter settings, demonstrating the effectiveness and robustness of the presented TO method.

Modeling the drying kinetics of soybeans under intermittent operation in thin layer

The extraordinary expansion of soybeans production and trade volumes over the past few decades has undoubtedly generated global economic benefits.One factor responsible for the commercial success and extension of soybean shelf life is the drying process, which involves both heat and mass transfer. Alternatively, intermittent drying aims to improve the process in terms of yield, energy use and product quality. Thus, presenting a mathematical model that describes the soybean drying kinetics in intermittent thin-layer operation is a novelty in this study. The first order Runge-Kutta method was used to solve the proposed model, through which it was possible to obtain a detailed knowledge of the heat and mass transfer parameters involved in the drying process. It was possible to properly fit a mathematical model, based on mass and energy balances, which predicted the temperature and moisture profiles of the grain as a function of drying time, as well as the determination of physical parameters, such as global coefficients natural and forced convection heat transfer and the apparent global mass transfer coefficient.

Numerical investigation of forced convection heat transfer from a sphere at low Prandtl numbers

Direct numerical simulations of the flow and forced convective heat transfer around a sphere at Reynolds numbers between Re=500 and Re=1000 are performed. We investigate the effects of the Prandtl number (Pr) on the forced convective heat transfer from a sphere for various fluids having Pr=0.01,0.1,0.7. At the larger Prandtl number, the convective transport due to the vortex shedding process dominates over the diffusive transport. As the Prandtl number decreases, diffusive effects become important. Moreover, the thermal boundary layer increases with decrements of the Prandtl number, which results in a reduction in the local and mean non-dimensional heat transfer coefficient. It is seen that at Re=500 and Re=750, the vortex shedding process is asymmetric, which results not only in a non-zero lift coefficient, but in an asymmetric temperature field in the wake of the sphere at Pr≥0.1. The dual asymmetry in the flow and the convective heat transfer is smooth out when the Prandtl number reaches Pr=0.01 as the heat diffusion dominates and asymmetries in the vortex formation zone are no longer relevant in the heat transport. The descend in Prandtl number also produces an attenuation of the temperature fluctuations and thereby, in the turbulent heat transfer. As a direct consequence, two factors emerge at Pr=0.01: (1) a lower decay ratio of the temperature in the wake centreline, and (2) a larger wake spread compared to higher Prandtl numbers.

Topology Optimization Design of Scramjet Structures With Forced Convective Heat Transfer on Unstructured Meshes

Abstract A topology optimization strategy with coupled fluid–solid interaction was proposed to maximize the cooling efficiency of a kind of structure applied for scramjets. The Galerkin finite element method (FEM) is used to solve the forced convective heat transfer, and the rational approximation of material properties (RAMP) method combined with the globally convergent method of moving asymptotes (GCMMA) method are used to solve the topological optimization models with different boundary conditions and objective functions. Examples are provided to demonstrate the validity and effectiveness of the optimization strategy. The optimal flow passages of scramjet structures are achieved successfully. Compared with a baseline structure with rectangular straight passages, the optimized flow passages significantly reduce the averaged bulk temperature and pressure loss, and the bulk temperature is more uniform to avoid the occurrence of concentrated high-temperature areas. With the Reynolds number changing from 1000 to 1750, the heat transfer performance of the three-dimensional topology-optimized structure increases by 16.79% to 20.82%.

Experimental study on the forced convection heat transfer characteristics of airflow with variable thermophysical parameters in a circular tube

The heat transfer of fluids in pipelines under conditions with variable thermophysical parameters is ubiquitous in engineering applications, but effective methods to predict the heat transfer parameters are scarce. To address this issue, an experimental method was proposed to measure the heat transfer parameters in a circular tube by considering the effect of the insufficient development of turbulence in the inlet region and a Robin boundary on the wall. The variation of the Nusselt number of the inner wall was calculated by measuring the temperature distribution of the outer wall, and a temperature-related Gnielinski correlation was developed for conditions under which the thermophysical parameters vary with the fluid temperature. The results demonstrated that the deviations between the experimental Nusselt numbers and the values predicted using the traditional Gnielinski correlation became larger and larger as the flow distance increased. The temperature-related correlation generally improved the prediction accuracy by more than 25%, and the accuracy was better with increasing inlet temperature and decreasing airflow velocity. This study provides an important empirical reference for the prediction of the convective heat transfer parameters in pipelines and related engineering applications under strong heating or cooling conditions.

Numerical Modelling of Forced Convection of Nanofluids in Smooth, Round Tubes: A Review

A comprehensive review of published works dealing with numerical modelling of forced convection heat transfer and hydrodynamics of nanofluids is presented. Due to the extensive literature, the review is limited to straight, smooth, circular tubes, as this is the basic geometry in shell-and-tube exchangers. Works on numerical modelling of forced convection in tubes are presented chronologically in the first part of the article. Particular attention was paid to the method of the solution of governing equations, geometry of the heating section, and boundary conditions assumed. Influence of nanoparticles on heat transfer and flow resistance are discussed. Basic information is summarized in tabular form, separately for single-phase approach and two-phase models. The second part of the article contains the correlation equations proposed in the presented papers for the calculation of the Nusselt (Nu) number or heat transfer coefficient, separately for laminar and turbulent flow. Details of the type of nanofluids, the concentration of nanoparticles, and the Reynolds (Re) number range are also presented. Finally, advantages and disadvantages of individual numerical approaches are discussed.

Two- and three-dimensional simulations of flow and heat transfer around rectangular cylinders

This numerical study investigates the forced convection heat transfer from and flow topology around isothermal rectangular cylinders. The effects of various cross-sectional aspect ratios (AR = 0.25–4), Reynolds numbers (Re = 30–200), and Prandtl numbers (Pr = 0.7, 5) are examined on the results. Two or three-dimensional simulations are conducted depending on the Re and AR employed. The results show that the near primary vortices (Kármán wake) undergo a downstream transition to the two-layered vortices, followed by a transition from the two-layered vortices to the secondary vortices for AR ≤ 1. For smaller aspect ratios, the prevailing of the secondary vortices is further accelerated, i.e., they emerge at a lower Re and a shorter downstream distance from the cylinder. Three types of secondary vortices distinguished based on their shape, strength, and generation mechanism were observed. A geometric criterion (spacing ratio) for the onset of evolution of the two-layer structure is proposed for each AR. It is observed that the sensitiveness of the Nusselt number to AR and Re can be ascribed to the change in the scenarios of the flow separation and reattachment (flow regimes), vortex strength, and wake-recirculation size (Lr). The Nusselt number grows with increasing Re and Pr but diminishes with AR. The relationship between the average Nusselt number and Lr is direct in the steady flow, while they are inversely linked in the two- and three-dimensional unsteady flow. Reducing AR from 4 to 0.25, depending on Re and Pr, leads to a 90%–170% enhancement in the Nusselt number, whereas increasing AR amplifies the total heat transfer due to an enlargement in the heat transfer surfaces. The second law of thermodynamics analysis reveals that cylinders with smaller AR generate lower entropy (destroy lower exergy); therefore, they are more efficient.

Experimental analysis of forced convection heat transfer of Hybrid Nanofluids in a vertical annulus with cosine heat flux

In this paper, the effect of hybrid Nanofluids on heat transfer of flow around a vertical rod with cosine heat flux is investigated. TiO2 and Al2O3 Nanoparticles are added to deionized water to produce hybrid Nanofluids. Experiments are conducted for both turbulence and laminar flow regimes. The test rig is an annulus in which a pitch-variable helical element applies a cosine heat flux to its inner tube. Results reveal that the increase in concentrations would reduce maximum rod temperature significantly. This is near l20% for 1.5%Al2O3 compared to water. The effect of pressure was also studied. The data obtained at pressures of 1, 2, 5, and 10 bar shows that pressure does not affect heat transfer performance. The thermal enhancement of Nanofluid decreases as the Reynolds number increases.

Wire-coil insert optimization for high-heat-load/flux synchrotron components

Many synchrotron components require high levels of internal-flow, forced-convection heat transfer to minimize surface temperatures, thermal gradients, and thermally induced stress on high-power beam-interacting surfaces. Wire-coil inserts, physically similar to a common spring, are mechanically fitted inside of component cooling passages to optimize heat transfer performance. They are routinely used in Advanced Photon Source (APS) front end and beamline high-heat-load/flux components to significantly enhance convection heat transfer—up to 400% compared to plain open passages. This has the additional benefit of greatly reducing coolant flow requirements for these components. Using several cooling passage sizes, five different wire sizes, and a range of pitch values, an experimental investigation conducted at the APS has determined the average heat transfer coefficient and resulting pressure loss as a function of water flow rate for 65 different wire-coil inserts. Data from this study have been non-dimensionalized and generalized to yield relationships that can be used to determine the heat transfer performance and resulting pressure loss for any given wire-coil insert that may be used at the APS. Through data reduction, the wire-coil insert characteristic dimensions have also been optimized to yield the highest heat transfer enhancement while minimizing the coolant flow requirements. These generalized expressions for wire-coil inserts will be presented, and they can be used by scientists and engineers during the component design process to evaluate achievable heat transfer performance and associated pressure loss, aiding in the establishment of optimized operating parameters and cooling passage flow distribution schemes.

Analysis of the effects of forced convective heat transfer to reduce the efficiency of heaters of gas pressure reducing stations

This study investigates the thermal modeling of heaters at the city gate stations (CGS) in Iran with a focus on efficient heat transfer and fuel consumption. Gas heaters are basic equipment with applications in various industries, especially CGS sites. Research on gas heaters and their optimal and efficient performance has been a challenging subject in the engineering world. This study aimed to change the heaters' thermal functionality from free to forced convection and examine the effect of such alteration on the heaters' efficiency while reducing their energy consumption. To make forced convection in the heater, a pump is required to be installed on it. This pump causes fluid circulation inside the heater and by creating movement in the fluid, it increases the heat transfer coefficient and changes the heat transfer mechanism of the system from free convection to forced convection. Using the first law of thermodynamics, the net energy these heaters emit into the environment consists of multiple components. These include the energy expended as follows: in the chimney, to warm up the gas, and the part dissipated away from the heater's body. The heat transfer coefficient is determined after calculating the energy loss for each of the components. Changes in the required energy for the total heat transfer coefficient for both free and forced convection may be analyzed. The results of this research indicate that forced convection heat transfer causes more heat loss from the heater's body than that transferred to the gas stream in the heater coils. This energy transfer ultimately reduces the heater's efficiency concerning the net generated heat. Thus, a reduction in the efficiency occurs and the initial efficiency (32%) of the free convection declines to a lower coefficient value for the forced convection.

Forced convection heat transfer in heat sinks with topologies based on triply periodic minimal surfaces

This study numerically and experimentally investigates the forced convection heat transfer in three mathematically designed heat sinks with lattice topologies based on triply periodic minimal surfaces (TPMS). The TPMS lattices consist of periodically arranged Diamond (D) or Gyroid (G) unit cells of 10 mm size and 80% porosity, considering two TPMS topologies; solid and sheet networks. This investigation examines the thermohydraulic performance of these novel heat sinks and compares them with other heat sinks in the literature. The hydrodynamic characteristics are studied with an airflow channel, and the thermal performance is explored with a validated numerical model. The results show that form drag dominates pressure drop in the range 2000 < Re < 7000. Due to the lowest surface area and largest pore size, the G-Solid heat sink reported the lowest friction factor ( f ). Concerning the thermal performance, G-Sheet showed the highest areal convection heat transfer coefficient ( hA ) and the lowest thermal resistance ( R th ) as a result of having the highest surface area. For a given pumping power, D-Solid exhibits the highest thermal efficiency ( η ). This work opens the door for designing novel 3D printable heat sinks and investigating their performance in thermal management systems.

Numerical simulation of forced convection heat transfer mechanism and comprehensive performance on hydrophobic structure surface

The numerical simulation of the hydrophobic structured surfaces is conducted in this work to investigate the confined effects of heat transfer performance and forced convection resistance. A new heat transfer model of air cavities is established based on the cavitation heat transfer process. Four structures (i.e., longitudinal ridges, square posts, square holes and transverse ridges) are constructed to study the effect of air cavities and slip-velocity on heat transfer. The goodness factor φ is utilized to evaluate the overall performance of the hydrophobic structured surfaces with different geometries. The results show that the existence of air cavities produces a negative effect of heat transfer, and the slip-velocity can enhance heat transfer performance. However, the coupling effect of air cavities and slip-velocity is related to surface structure rather than linear superposition. In addition, the heat transfer performance and frictional resistance, as well as goodness factor are dependent on the structure and contact angle of hydrophobic surface. Compared with the smooth surface, the hydrophobic surfaces for all structures exhibit a better goodness factor, and the maximum φ/φ0 of 1.105 can be achieved on the square posts structure.

Forced convection heat transfer: A comparison between open-cell metal foams and additive manufactured kelvin cells

The Kelvin cells are normally applied to represent the open-cell metal foams for simulation due to the similar pore structure. However, the experimental comparison between Kelvin cells and open-cell metal foams is rarely reported. This study compares the forced convective heat transfer properties of four Ni70Cr30 metal foams and one GH4169 Kevin cells manufactured from additive manufacturing. The results indicate that the pressure drop and overall heat transfer coefficient (HTC) increase with the increase of PPI, while decreasing with the increase of porosity. Moreover, the comparisons between present data and data from the literature prove the reasonability of the experiments. The pressure drop and overall HTC of Kelvin cells are 64.4% and 12.3% higher than open-cell metal foams, while showing 26.2% lower overall heat transfer performance at 6 m/s. The pore structure, thicker skeleton, higher porosity, and surface roughness of Kelvin cells contribute to the higher pressure drop than open-cell metal foams. The comparison among experimental correlations and present data indicates that the correlations for predicting the behavior of open-cell metal foams could not be used directly to predict Kelvin cells.