Effect Of Heat Flux Distribution Research Articles

Time-dependent analysis of heat transfer enhancement and entropy generation of hybrid nanofluids in a tube with a solid and elliptical‑cut twisted tape insert with non-uniform heat flux

ABSTRACT This numerical study investigates the improvement of transient heat transfer in a plain tube using hybrid nanofluids consisting of solid particles and twisted tapes with elliptical cuts, under non-uniform heat flux conditions. The analysis includes the calculation of the Bejan number and entropy production. Different working fluids, including water, CuO/water nanofluid, and a hybrid nanofluid with a 2% volume concentration of Al2O3-Cu/water, are used with varying concentrations of the hybrid nanofluid ranging from 1% to 4%. Effects of heat flux distribution and local concentration ratio (LCR) are investigated. The computational results indicate that conventional and elliptical cut twisted tapes enhance transient heat transmission. A slight rise in the heat transfer coefficient is observed when the fluid has higher thermal conductivity. The flow velocity gradually stabilizes over time. The hybrid nanofluid of (Al2O3-Cu/water) significantly affects transient heat transmission, reducing the maximum temperature difference by approximately 4.1% compared to water and 6.2% compared to the nanofluid. Transient heat transmission is further intensified by TECT. Moreover, frictional entropy production dominates the system’s irreversibility. This study contributes to the understanding of transient heat transfer enhancement and its dependence on hybrid nanofluids, providing insights for engineering applications.

Effect of Heat Flux Distribution on Entropy Generation and Irreversibility of Lead-Bismuth Eutectic (LBE) Forced Convective Heat Transfer

Effect of Heat Flux Distribution on Entropy Generation and Irreversibility of Lead-Bismuth Eutectic (LBE) Forced Convective Heat Transfer

The effect of heat flux distribution and internal heat generation on the thermal damage in multilayer tissue in thermotherapy.

Proper analysis of the temperature distribution during heat therapy in the target tissue and around it will prevent damage to other adjacent healthy cells. In this study, the exact solution of steady and unsteady of the hyperbolic bioheat equations is performed for multilayer skin with tumor at different heat fluxes on its surface and the generation of internal heat in the tumor. By determining the temperature distribution in three modes of constant heat flux, parabolic heat flux and internal heat generation in tumor tissue, the amount of burn in all three modes is evaluated. The results indicated that the Fourier or non-Fourier behavior of tissue has no role in the rate of burns in thermotherapy processes. At equal powers applied to the tissue, the internal heat generation in the tumor, constant flux and parabolic flux on the skin surface have the most uniform and most non-uniform temperature distribution, respectively and cause the least and the most thermal damage in the tissue.

Flow and heat transfer simulation in porous volumetric solar receivers by non-orthogonal multiple-relaxation time lattice Boltzmann method

Flow behavior and heat transfer (FBHT) process within porous volumetric solar receivers (VSR) were studied numerically by the non-orthogonal Multiple-Relaxation Time Lattice Boltzmann Method (NO-MRT-LBM). Three types of porous structures within VSR were distributed and studied. A non-uniform Gaussian heat influx distribution was adopted to approximate the actual conditions. The effects of pore structure (types 1-3) and heat flux distribution (w) on flow behavior and heat transfer process were studied. It was found that the effect of heat flux distribution on the internal heat transfer process of VSR with different porous structure types was mainly determined by the inlet distribution of solid particles. In type 1 and type 2, the instantaneous temperature growth rate of solid particles increased with decreasing w at the inlet area of the model, thus the maximum temperature, the average temperature of the solid particles within the receiver and the air at the outlet increased rapidly. The average temperature of solid particles and air at the outlet in type 3 both increased with the increase of w. Compared with type 2 and type 3, type 1 showed the best effect of heat transfer when heat flux distribution tended to be uneven, which, however, needs to be reduced to avoid the local temperature becoming excessively large. When heat flux distribution tended to be uniform, type 3 showed better heat transfer performance where any excessive local temperature was eliminated.

Optimization of thermo-hydraulic characteristics of solar cavity receiver under concentrated heat flux.

It is important to study the effects of heat flux on the thermo-hydraulic characteristics in a solar cavity receiver because of the non-uniform radiation flux temporally and spatially. In this article, we presented a mathematical model of thermo-hydraulic characteristics of a solar cavity receiver, considering the effect of heat flux distribution on the energy transfer (radiation-conduction-convection). Using the model, the thermo-hydraulic characteristics under high concentrated heat flux were studied and then optimized the characteristics from two aspects: tube diameter (22, 27, 32, and 38 mm) and connection structure of the heating surface (H-type, central inlet/outlet, and vertical U-type). It was found that flow distribution changed smoothly at the diameter of 27 mm with the increase of the heat flux; when the diameter of tubes at the certain distance (1.6σHF) from the spot center was replaced by 38 mm, the thermo-hydraulic characteristics were improved. For the evaporating surfaces, the thermo-hydraulic characteristics of working fluid (water) with the central inlet/outlet connection structure were better than those of the H-type connection structure. For the surperheated surfaces, the vertical U-type connection structure was applied to obtain the high temperature steam. These research findings are helpful for the safe and stable operation of the whole solar power system.

Investigation of Optimum Heat Flux Profile Based on the Boiling Safety Factor

An experimental study is conducted to investigate the effect of heat flux distribution on the boiling safety factor of its cooling channel. The water is allowed to flow in a horizontal circular pipe whose outlet surface is subjected to different heat flux profiles. Four types of heat flux distribution profiles are used during experiments: (constant distribution profile, type a, triangle distribution profile with its maximum in channel center, type b, triangle distribution profile with its maximum in the channel inlet, type c, and triangle distribution profile with its maximum in the channel outlet, type d). The study is conducted using heat sources of (1000 and 2665W), water flow rates of (5, 7 and 9 lit/min). The water temperature at cooling channel inlet is kept constant at (25°C). Copper test section of (0.6 m) length (0.025m) inner diameter is used during the experiments. The electrical heater used for water heating is wrapped around the copper pipe covering (50 cm) of its length. Calibrated thermocouples are distributed along pipe surface at distances (0.1, 0.2, 0.3, 0.4 and 0.5 m) from pipe inlet to measure pipe surface temperature. The results shows that the heat source with heat flux profile of type (c) is the most reliable one from thermo-hydraulic safety point of view for both types of heat sources, as it ensures a maximum boiling safety factor (K) of (1.6, 1.7, 2) at water flow rates of (5, 7 and 9 lit/min) respectively based on maximum heat capacity of (2665 w), while the heat source with heat flux profile of type (d) which posses minimum boiling safety factors of (1, 1.2, 1.3) at water flow rates of (5, 7 and 9 lit/min) respectively based on same heat capacity value is the worst one from same point of view.

Natural convection of a nanofluid inside a vertical circular enclosure exposed to a non-uniform heat flux

In the present study natural convection of Al2O3-water nanofluid inside a vertical circular enclosure is investigated. The enclosure is exposed to heat flux with a non-uniform distribution from the bottom. The objective is finding the heat flux distribution for which the heat transfer coefficient is optimum at different values of the governing parameters. For examining the effect of heat flux distribution on thermal transport phenomenon, 9 various flux distributions are considered as cases 1 to 9. The bottom side of the enclosure is divided to six annular strips each of which receives a portion of the total heat i.e., the flux applied on each strip is different. The side wall is kept at constant temperature and the top wall is assumed to be adiabatic. A numerical finite volume scheme is employed to solve non-dimensional conservative equations in cylindrical coordinates. In addition to the heat flux distribution, the effects of Rayleigh number (104<Ra<106), nanoparticles volume fraction (φ<0.08) and aspect ratio (Γ=0.5, 1 and 1.5) on isotherms and Nusselt number are analyzed. It is observed that there are physical conditions for which the heat transfer characteristic is optimized.

Thermochemical performance analysis of solar driven CO2 methane reforming

Increasing CO2 emission problems create urgent challenges for alleviating global warming, and the capture of CO2 has become an essential field of scientific research. In this study, a finite volume method (FVM) coupled with thermochemical kinetics was developed to analyze the solar driven CO2 methane reforming process in a metallic foam reactor. The local thermal non-equilibrium (LTNE) model coupled with radiative heat transfer was developed to provide more temperature information. A joint inversion method based on chemical process software and the FVM coupled with thermochemical kinetics was developed to obtain the thermochemical reaction parameters and guarantee the calculation accuracy. The detailed thermal and thermochemical performance in the metal foam reactor was analyzed. In addition, the effects of heat flux distribution and porosity on the solar driven CO2 methane reforming process were analyzed. The numerical results can serve as theoretical guidance for the solar driven CO2 methane reforming application.

Experimental and numerical heat transfer analysis of a V-cavity absorber for linear parabolic trough solar collector

In the present study, a V-cavity absorber with rectangular fins that can be used in the linear parabolic trough collector (PTC) system was proposed and investigated both theoretically and experimentally. The optical performance of the absorber was studied by means of Monte-Carlo ray-tracing (MCRT) method. An energy balance model and a more detailed three-dimensional numerical model were developed to analyze the flow and heat transfer characteristics. Moreover, an experimental setup was built to validate the theoretical analysis. A reasonable agreement between the theoretical and experimental results was achieved, which proves the feasibility and reliability of the models. The results show that the sunlight could be reflected repeatedly by the triangle shape and nearly no sunlight escapes. The absorber with rectangular fins has a better heat transfer performance with higher outlet temperature of heat transfer fluid (HTF), lower temperature of heating surface and lower heat loss than those of absorber without fins. The effects of heat flux distribution, mass flow rate and direct normal irradiance on the heat transfer performance were further discussed. In addition, the variations of the heat transfer coefficient along z axial direction with different mass flow rates were also calculated based on the numerical results.

An experimental study of critical heat flux in non-uniformly heated vertical annulus under low flow conditions

An experimental study on critical heat flux (CHF) has been performed in an internally heated vertical annulus with non-uniform heating. The CHF data for the chopped cosine heat flux have been compared with those for uniform heat flux obtained from the previous study of the authors, in order to investigate the effect of axial heat flux distribution on CHF. The local CHF with the parameters such as mass flux and critical quality shows an irregular behavior. However, thetotal critical power with mass flux and theaverage CHF with critical quality are represented by a unique curve without the irregularity. The effect of the heat flux distribution on CHF is large at low pressure conditions but becomes rapidly smaller as the pressure increases. The relationship between the critical quality and the boiling length is represented by a single curve, independent of the axial heat flux distribution. For non-uniform axial heat flux distribution, the prediction results from Doerffer et al.’s and Bowring’s CHF correlations have considerably large errors, compared to the prediction for uniform heat flux distribution. KeyWords : Critical Heat Flux, Heated Vertical Annulus, Low Mass Flux, Wide Range Pressure, Non-uniform Heating, Effect of Axial Heat Flux Distribution, Boiling Length

An analysis of peripheral heat transfer in man.

An analysis of peripheral heat transfer in man.