Forced Convection In Channel Research Articles

Optimized design of multiple vortex generator rows to enhance thermo-hydraulic performance in fully developed forced convection channel

In this study, optimized design schemes for improving the thermo-hydraulic performance of delta-winglet vortex generators (VGs) arranged in multiple rows in fully developed forced-convection channel are investigated numerically for a range of Re numbers from 5000 to 20,000. The height ratio (HR) of VGs with respect to the channel and the longitudinal pitch ratio (PR) of VG arrangement are considered as the main parameters for the design of multiple rows of VG, and a comprehensive investigation was carried out from the aspects of thermal performance, flow characteristics, entropy generation rate and energy saving. For better thermal performance, HR = 1/6 and PR = 16 are the most recommended design parameters (PEC = 1.33–1.38), and it is necessary to avoid design parameters with too small a PR or HR. From the local flow field analysis, the multiple rows arrangement of VG can promote the diffusion and enhancement of the secondary flow to realize the enhancement of thermal mixing along the flow direction. Finally, the entropy generation rate is analyzed to assess the irreversibility level for different design parameters, the optimized multiple row arrangement design of VG is given by the performance evaluation plot in terms of energy saving considerations.

Direct numerical simulation of the fully developed turbulent free convection flow in an asymmetrically heated vertical channel

We report direct numerical simulations of the fully developed turbulent natural convection flow of a Boussinesq fluid (Pr = 0.71) in a vertical channel. One wall is heated with a constant heat flux and the other is assumed to be perfectly insulated. Simulations were performed at three different Rayleigh numbers (Ra = 5 × 105, 106 and 5 × 106). Predictions of the time averaged velocities, temperatures and intensities of the fluctuations agree with experiments reported in the literature. Under the fully developed flow conditions considered in this study the shear stress is greater on thermally active wall than that on the adiabatic wall, but the turbulent intensities and the turbulent shear stress are greater near the insulated wall. Conversely, the intensities of the temperature fluctuations and turbulent heat fluxes are higher near the thermally active wall than those near the adiabatic wall. Contrary to the vertical channel with constant but different wall temperatures, where buoyancy and turbulent shear dominate the generation of turbulent kinetic energy, in the asymmetrically heated channel the turbulent shear stress is the principal mechanism of turbulent kinetic energy production as in a forced convection channel flow.

Investigation of the Effect of Perforated Sheath on Thermal-Flow Characteristics Over a Gas Turbine Reverse-Flow Combustor—Part 1: Experiment

Abstract Reverse-flow combustors have been used in heavy, land-based gas turbines for many decades. A sheath is typically installed over the external walls of the combustor and transition piece to provide enhanced cooling through hundreds of small impinging cooling jets, followed by a strong forced convection channel flow. However, this cooling is at the expense of a large pressure loss. With the modern advancements in metallurgy and thermal-barrier coating technologies, it may become possible to remove this sheath to recover the pressure loss without causing thermal damage to the combustor chamber and the transition piece walls. However, without the sheath, the flow inside the dump diffuser may exert nonuniformly reduced cooling on the combustion chamber and transition piece walls. The objective of this paper is to investigate the difference in flow pattern, pressure drop, and heat transfer distribution in the dump diffuser and over the outer surface of the combustor with and without a sheath. Both experimental and computational studies are performed and presented in Part 1 and Part 2, respectively. The experiments are conducted under low pressure and temperature laboratory conditions to provide a database to validate the computational model, which is then used to simulate the thermal-flow field surrounding the combustor and transition piece under elevated gas turbine operating conditions. The experimental results show that the pressure loss between the dump diffuser inlet and exit is 1.15% of the total inlet pressure for the non-sheathed case and 1.9% for the sheathed case. This gives a 0.75 percentage point (or 40%) reduction in pressure losses. When the sheath is removed in the laboratory, the maximum increase of surface temperature is about 35%, with an average increase of 13–22% based on the temperature scale of 23 K, which is the difference between the bulk inlet and the outlet temperatures.

Effect of mixed hydrophilic and hydrophobic surface coatings on droplets freezing and subsequent frost growth during air forced convection channel flows

The formation of frost and ice decreases the energy efficiency of refrigeration systems and of air-source heat pump systems. This paper presents new experimental data of frost nucleation and frost growth on cold flat plates operating in frosting conditions with air forced convective flow. The plates surfaces had different wettability: from an uncoated aluminum surface with fine finish roughness and contact angle of 75°, to hydrophobic (θ ≈ 110–116°) and hydrophilic (θ ≈ 19–29°) coatings. Three biphilic coatings with regions of hydrophilic coatings adjacent to areas with hydrophobic coatings were investigated. These coatings manage water droplets movements, reduce freezing temperatures, and delay icing and onset of frost nucleation. A new experimental technique was developed in order to mimic actual field type operating conditions of convective channel flow experienced by fin structures of heat exchangers of typical air-source heat pump systems. Hydrophobic surface and one biphilic surface had early phase changeovers and high thresholds of the frost thickness before switching to the frost growth phase due to the presence of large droplets on the surfaces before they froze into ice beads. Fine-finished aluminum surface and hydrophilic coating had delayed phase changeovers and low frost thicknesses with respect to the other surfaces.

Optimal and Experimental Analysis of Electrohydrodynamic Enhancement of Water Evaporation

The paper investigates the EHD effect on the evaporating rate of the water surfaces by numerical and experimental methods. Two-dimensional turbulent forced convection channel flow is theoretically analyzed. Parametric evaluation including the applied voltage, electrode location is executed in details. The related optimization of the electrode pitch and height for specified input voltage is investigated numerically along with a simplified conjugated-gradient method (SCGM). The results show the EHD effect on the evaporating rate is increased with increase of voltage and electrode pitch and decrease of electrode height. In addition, based on the per unit power consumption, the best mass transfer performance is obtained at the optimal electrode location for each input voltage case. At last, the numerical results get a good consistency with the experiment within a discrepancy of 27%.

Performance optimization of a channel flow problem using shape functions

This study explores the use of shape functions on the boundary condition optimization of a forced convection channel flow subjected to an axial heat flux distribution. The goal is to determine the heat flux distribution that minimizes the coolant overheating and simultaneously reduces the computational optimization efforts. The calculations are implemented in a 2-D homemade code, which solved the conservation equations of mass, momentum and energy, while coupling the optimization variables, here represented by the multiple coefficients of the shape functions, with a genetic algorithm. Generally speaking, two types of shape functions were used: unbiased and biased. In the former, the optimization procedure is responsible for obtaining the optimal coefficients for Legendre shape functions, while in the latter, scaling-based power laws are used to construct shape functions. The results computed for both biased and unbiased shape functions show that not only higher performance levels (i.e., less overheating) can be obtained with the present formulation when compared with the techniques employed so far in the available literature (e.g., constant discrete heat flux heaters along the channel), but it also significantly reduced computation efforts. More specifically, the biased shape functions outperform the unbiased formulation by lowering the maximum plate temperature as much as 20% with respect to the standard constant heat flux case, while simultaneously reducing the computational time by a factor of over 4.

Heat transfer enhancement of laminar forced convection in a channel by von-Karman vortex generator

The work deals with a method of enhancing laminar, forced convection in channels. It is well-known that the inadequate flow mixing process in laminar flow is the limiting factor for heat transfer compared with the turbulent flow. In this work, vortex shedding (von-Karman vortex) technique is used to enhance the rate of heat transfer in a channel fitted with off-centre obstacles. Vortex formations at the downstream of the obstacles enhance the fluid mixing process; consequently, enhancing the rate of heat transfer. The distance between obstacles is selected based on fact that at certain the downstream distance of the obstacle, the strength of the vortex decays. Lattice Boltzmann method is used to solve the governing equations.

Direct numerical simulation of turbulent forced convection in a wavy channel at low and order one Prandtl number

Turbulent forced convection in a channel with one planar wall and one wall of sinusoidal shape is investigated by Direct Numerical Simulation. The flow is fully developed and the Reynolds number based on the mean bulk velocity and the average hydraulic diameter is Re ≈ 18,900; in this weakly turbulent flow regime three different Prandtl number values are investigated, Pr = 0.025, 0.20, 0.71. The fluid is in contact with the colder channel walls at an equal, uniform temperature. The main statistical quantities, like the root-mean-square of temperature fluctuations and the turbulent heat fluxes, the local heat transfer coefficient and turbulent Prandtl number values are reported. Effects of flow separation and reattachment on the local heat transfer rate and turbulent Prandtl number distribution are also presented and discussed.An a priori analysis of the behavior of the simple gradient diffusion model of turbulent heat fluxes is performed in the low Prandtl number, separated flow conditions of the present work. While the low Prandtl number effect can be accounted for by an appropriate selection of the turbulent Prandtl number value to be provided to the model, deviations form the expected behavior of turbulent heat fluxes are seen to occur in the flow separation region and downstream reattachment.

Constructal flow orientation in conjugate cooling channels with internal heat generation

This paper presents the development of the three-dimensional flow architecture of conjugate cooling channels in forced convection with internal heat generation within the solid for an array of circular cooling channels with different flow orientation. Three flow orientations were studied: array of channels with parallel flow; array of channels in which the flow in every second row is in a counter direction with its neighbours, and flows in all the arrays of channels are in counter flow relative to each other. The geometric configurations were determined in such a way that the peak temperature was minimised subject to the constraint of fixed global volume of solid material. The degrees of freedom of the design were hydraulic diameter and channel to channel spacing. A gradient-based optimisation algorithm was applied to search for the best optimal geometric configurations that improve thermal performance by minimising thermal resistance for a wide range of dimensionless pressure differences. The effect of porosities, applied pressure difference, flow orientation and heat generation rate on the optimal hydraulic diameter and channel to channel spacing is reported. The results show that the effects of dimensionless pressure drop on minimum thermal resistance were consistent with those obtained in the open literature.

Constructal optimisation of conjugate triangular cooling channels with internal heat generation

This paper presents the development of the three-dimensional flow architecture of conjugate cooling channels in forced convection with internal heat generation within a solid. Two types of cross-section channel geometries were used. The first involved equilateral triangles with three equal legs in length and all three internal angles of 60°. The second was isosceles right triangles with two legs of equal length and internal angles of 90°, 45° and 45°. Both the equilateral triangle and isosceles right triangle are special case of triangle that can easily and uniformly be packed and arranged to form a larger constructs. The configurations were optimised in such a way that the peak temperature of the heat generating solid was minimised subject to the constraint of a fixed global volume of the solid material. The cooling fluid was driven through the channels by the pressure difference across the channel. The degrees of freedom of the channels were aspect ratio, hydraulic diameter and channel to channel spacing ratio. The shape of the channel was allowed to morph to determine the best configuration that gives the lowest thermal resistance. A gradient-based optimisation algorithm was applied in order to search for the best optimal geometric configurations that improve thermal performance by minimising thermal resistance for a wide range of dimensionless pressure difference. The effects of porosities, applied pressure and heat generation rate on the optimal aspect ratio and channel to channel spacing are reported. It was found that there are unique optimal design variables for a given pressure difference. The numerical results that were obtained were in agreement with the theoretical formulation using scale analysis and method of intersection of asymptotes. Results obtained show that the effects of applied dimensionless pressure drop on minimum thermal resistance were consistent with those obtained in the open literature.

Constructal conjugate cooling channels with internal heat generation

This paper presents a geometric optimisation of conjugate cooling channels in forced convection with internal heat generation. Two configurations were studied; circular channels and square channels. The configurations were optimised in such a way that the peak temperatures were minimised subject to the constraint of fixed total global volume. The fluid was forced through the cooling channels by the pressure difference across the channels. The structure has one degree of freedom as design variable: channel hydraulic diameter and once the optimal channel hydraulic diameter is found, optimal elemental volume and channel-to-channel spacing result. A gradient-based optimisation algorithm is applied in order to search for the best and optimal geometric configurations that improve thermal performance by minimising thermal resistance for a wide range of dimensionless pressure difference. This optimiser adequately handles the numerical objective function obtained from CFD simulations. The results obtained show the behaviour of the applied pressure difference on the optimised geometry. There are unique optimal design variables for a given pressure difference. The numerical results obtained are in agreement with the theoretical formulation using scale analysis and method of intersection of asymptotes.

Mathematical optimisation of laminar forced convection heat transfer through a vascularised solid with square channels

This paper presents a three-dimensional geometric optimisation of cooling channels in forced convection of a vascularised material with the localised self-cooling property subjected to a heat flux. A square configuration was studied with different porosities. Analytical and numerical solutions were provided. The geometrical configuration was optimised in such a way that the peak temperature was minimised at every point in the solid body. The optimisation was subject to the constraint of a fixed global volume of solid material, but the elemental volume was allowed to morph. The solid material was subject to a heat flux on one side and the cooling fluid was forced through the channels from the opposite direction with a specified pressure difference. The structure had three degrees of freedom as design variables: the elemental volume, channel hydraulic diameter and channel-to-channel spacing. A gradient-based optimisation algorithm was used to determine the optimal geometry that gave the lowest thermal resistance. This optimiser adequately handled the numerical objective function obtained from numerical simulations of the fluid flow and heat transfer. The numerical results obtained were in agreement with a theoretical formulation using scale analysis and the method of intersection of asymptotes. The results obtained show that as the pressure difference increases, the minimised thermal resistance decreases. The results also show the behaviour of the applied pressure difference on the optimised geometry. The use of the optimiser made the numerical results to be more robust with respect to the optimum internal configurations of the flow systems and the dimensionless pressure difference.

NUMERICAL INVESTIGATION ON CONVECTIVE HEAT TRANSFER IN HIGH TEMPERATURE SOLAR RECEIVER

In recent years, solar receivers have been employed in several applications where high temperature fluids are involved, such as power plants and solar thermal reactor. The possible presence of high temperature gradients could determine reduced receiver efficiency and a short life of the material. Thermal and fluid dynamic analysis of the receiver allows to highlight possible presence of temperature gradients and to evaluate their values and the more critical zone of the material. Compressibility effects are encountered in gas flows at high velocity and/or in which there are large pressure variations. In the present paper a numerical study on forced convection in the solar receiver vessel is carried out. In particular, the investigation is accomplished to evaluate the convective heat transfer behaviour in a non-straight rectangular channel as a function of hydraulic diameter and mass flow rate. The investigation is obtained in three-dimensional steady state turbulent forced convection in channels with Reynolds number ranging between 6600 and 13200. The fluid is carbon dioxide and its properties are assumed to be temperature dependent. A comparison between incompressible and compressible flows is also carried out. Results are presented in terms of temperature, pressure and velocity fields, average heat transfer coefficients, and pressure drops. It is observed that higher temperature values are reached near the inclined channel walls in correspondence of the stagnation regions. In addition a Von Karman wake with swirling vortexes are present downstream the duct corners.

Forced convection in a channel partly occupied by a bidisperse porous medium: Asymmetric case

This paper presents an analytic investigation of forced convection in parallel-plate channel partly occupied by a bidisperse porous medium and partly by a fluid clear of solid material, the distribution being asymmetrical. The walls of the channel are subject to an uniform heat flux; the flow is assumed to be hydrodynamically and thermally fully developed. The layer of a bidisperse porous medium is attached to one of the channel walls; it is modeled utilizing a two-velocity two-temperature formulation using Darcy’s law. The Beavers–Joseph boundary condition is employed at the bidisperse porous medium/clear fluid interface. The dependences of the Nusselt number on a conductivity ratio, a velocity ratio, a volume fraction, internal heat exchange parameter, and the position of the porous-fluid interface are investigated. Both cases of symmetric and asymmetric heating are investigated, which is specified by the asymmetry heating parameter introduced here. For the case of asymmetric heating, a singular behavior of the Nusselt number is found and explained.

A bioheat transfer model: Forced convection in a channel occupied by a porous medium with counterflow

An illustrative model for bioheat transfer is developed. An analytical solution is obtained for forced convection in a parallel plate channel occupied by a layered saturated porous medium with counterflow, the dominant feature that distinguishes bioheat transfer from other forms of heat transfer. The case of asymmetrical constant heat-flux boundary conditions is considered and the Brinkman model is employed for the porous medium. It is found that the Nusselt number Nu is zero when the mean velocity is zero, and negative values can be attained.

Thermally developing forced convection in a channel occupied by a porous medium saturated by a non-Newtonian fluid

The classical Graetz methodology is applied to investigate the thermal development of forced convection in a parallel plate channel filled by a saturated porous medium, with walls held at constant temperature, for the case of a non-Newtonian fluid of power-law type. A Brinkman-Forchheimer model is used for the momentum equation. The analysis for the case of small modified Darcy number leads to expressions for the local Nusselt number and average Nusselt number as functions of the dimensionless longitudinal coordinate, the power-law index, a modified Darcy number, and a modified Reynolds-Forchheimer number (with the last three parameters being involved via a boundary-layer thickness).

Thermal Equilibrium in Transient Forced Convection Porous Channel Flow

The validity of the local thermal equilibrium assumption in the transient forced convection channel flow is investigated numerically. Axial conduction in both fluid and solid domains is included. It is found that five dimensionless parameters control the local thermal equilibrium assumption. These parameters are the thermal diffusivity ratio αR, the volumetric Nusselt number Nu, the dimensionless channel length ξmax, Peclet number Pe, and the solid to fluid total thermal capacity ratio CR. The qualitative and quantitative aspects of the effects of these five parameters on the channel thermalization time are investigated.

Effects of Viscous Dissipation and Flow Work on Forced Convection in a Channel Filled by a Saturated Porous Medium

Fully developed forced convection in a parallel plate channel filled by a saturated porous medium, with walls held either at uniform temperature or at uniform heat flux, with the effects of viscous dissipation and flow work included, is treated analytically. The Brinkman model is employed. The analysis leads to expressions for the Nusselt number, as a function of the Darcy number and Brinkman number.

THERMAL EQUILIBRIUM IN TRANSIENT CONJUGATED FORCED-CONVECTION CHANNEL FLOW

The validity of the local thermal equilibrium assumption in transient conjugated forced-convection channel flow is investigated numerically. Axial conduction in both fluid and solid domains is included. It is found that five dimensionless parameters control the local thermal equilibrium assumption. These parameters are the thermal diffusivity ratio f R , the Biot number Bi, the dimensionless channel length \\xi_{\\max} , the Peclet number Pe, and the solid-to-fluid total thermal capacity ratio C R . The qualitative and quantitative aspects of the effects of these five parameters on the channel thermalization time are investigated.

On forced convection in channels partially filled with porous substrates

This paper numerically simulates the forced convection flow in the developing region of a parallel-plate channel partially filled with two porous substrates of equal thickness deposited at the inner walls of the channel. The major objective of the present work is to investigate the impact of several operating and design parameters on the thermal performance of the channel under consideration. The physical problem is simulated by using Darcy–Brinkman–Forchheimer model. For a prescribed amount of porous material, the current investigation discusses the comparison between inserting this entire amount at one side of the channel and inserting half of this amount at each side of the channel.