Non-uniform Wall Temperature Research Articles

Experimental and numerical studies of the fuel concentration distribution within the near-wall area in a compact space for a gas turbine combustor

Interstage combustion is known for its small axial distance, high combustion efficiency and low lean blowout boundary. However, in a compact space, the distance between the fuel injection location and the wall is confined, and the high-temperature wall may affect the fuel distribution and in turn affect the combustion efficiency. In this study, an experiment of kerosene single-droplet evaporation was conducted, and the fuel distribution in an interstage combustor was numerically simulated. The experimental results showed that the evaporation rate of fuel droplets decreased with increasing distance from the high-temperature wall, and the influence of the lower wall on fuel evaporation was greater than that of the sidewall. The fuel evaporation rate at the high-temperature position within the nonuniform wall temperature field was relatively high. The numerical simulation results indicated that with increasing temperature, the effect of the wall on fuel evaporation increased, as did the uniformity of the fuel distribution. The direction of the temperature gradient imposed the greatest effect on the fuel distribution under nonuniform temperature conditions. This will facilitate the selection of the optimal ignition location within the cavity to achieve efficient combustion. Moreover, it's possible to control the fuel distribution by adjusting the wall temperature.

Impact of impinging jet ventilation on thermal comfort and aerosol transmission: A numerical investigation in a densely-occupied classroom with solar effect

Urgent demands for ensuring safe and healthy spaces for children have been raised due to the post-COVID-19 pandemic. This study numerically modelled a densely populated classroom with 40 students and one teacher across two layouts with different relative vent positions to assess the effectiveness of an emerging ventilation scheme, impinging jet ventilation (IJV), in balancing thermal comfort and controlling indoor transmission. The effect of solar radiation was carefully analysed. Cough-induced contaminants were expelled from the infectious teacher and were traced by the Eulerian-Lagrangian approach. The exposure risk of students was further assessed via the inhalation index (ID) based on the spatial characteristics of the released particles. The thermal comfort was evaluated by the Fanger model. The results demonstrated that IJV with Layout 2 (teacher at the supply inlet and exhaust side) was found more optimal in counterbalancing the occupants’ thermal comfort and indoor transmission control. This layout not only aligned better with ASHRAE standards for thermal comfort, showing a reduced vertical thermal gradient but also lowered student exposure risk by 18.9%–135.2%. With solar radiation, heat absorbed by the windows led to non-uniform wall temperature distributions, causing the air near the windows to also exhibit a non-uniform profile. This interaction with the cooler air from the IJV system was found to facilitate contaminant dispersion. This resulted in a 10.3% to 26% increase in students’ average exposure risk compared to those without solar radiation cases. This study aimed to investigate the practical applications of IJV system for densely-occupied classrooms.

A numerical study of the effects of jet-aft wall temperatures on the dynamics of jets in hypersonic crossflows

For high-speed vehicles such as scramjets, internal combustion chamber temperatures play an important role in the engine performance, with the influence of the temperature on the fuel injection dynamics being of key interest. In this study, large eddy simulations are employed to investigate a sonic jet in a Mach 5 crossflow with a momentum flux ratio of 5.8 and the parametrization of the temperature of the wall aft of the jet. Both uniform and non-uniform wall temperatures are analyzed, with two jet-to-crossflow temperature ratios of 8.06 and 3.23 investigated. It is found that the wall temperature primarily influences the near wall flow, with a small amount of entrainment into the jet plume via the counter-rotating vortex pair as the low velocity flow is limited by the near-wall shear layer. It is found that the aft-recirculation zone is expanded with the increasing wall temperature, which has the effect of increasing the penetration of the jet plume into the far field. Five recirculation regions are observed ahead of the jet, which are noted to result from the interaction between the crossflow and jet flow for both the adiabatic and temperature-controlled cases, with jet fluid flowing into the forward boundary layer, and thus near-wall mixing is observed. Horseshoe vortex strength is seen to dissipate when passing over the cooled walls, thus reducing the mixing potential near the wall, where the opposite is true for heated walls. Lateral spread of the horseshoe vortices is seen to increase with cooled walls, increasing the near-wall mixing potential.

Numerical thermal augmentation of ternary nanofluid in a tube with stent, torus-ring and surface-grooved twisted tapes under non-uniform wall temperature

This paper presents a numerical analysis of flow and thermal characteristics in a plain tube with various inserts, including a stent, torus ring, and surface-grooved twisted tape, using a ternary nanofluid consisting of SiO2, ZnO, and CaO nanoparticles as the working fluid. Computational fluid dynamics simulations were conducted using ANSYS Fluent commercial codes, focusing on laminar Reynolds numbers ranging from 500 to 2000 and a non-uniform wall temperature profile. The objective is to investigate the Nusselt number, pressure drop, thermal performance, and pumping power of the ternary nanofluid with nanoparticle concentrations ranging from 0.01 to 0.05%, considering various geometrical parameters of the grooved twisted tape insert, such as thickness ratio (0.019–0.033), width ratio (0.38–0.68), and pitch ratio (1.77–5.77). The results show that flow and thermal indicators such as Nusselt number, thermal performance factor, pressure drop and pumping power increased with thickness and width ratios. Conversely, an increase in pitch ratio results in lower flow and thermal indicators. For instance, in a plane tube without inserts, no noticeable difference in pressure drop is observed and the thermal performance factor is about 1.87-1.25 times that of pure water when nanoparticle concentration is increased from 0.01 to 0.05%. The width ratio 0.68, thickness ratio 0.033 and pitch ratio 1.77 yielded a maximum thermal performance factor of about 1.4, 1.7 and 2.3 more than smooth tube, respectively, and a maximum pumping power 0.000654 W, 0.000223 W and 0.00020 W, respectively. The findings emphasize significant improvements in heat transfer and flow properties, shedding light on the innovative configuration's potential application as an improved heat exchanger. This research advances understanding of ternary nanofluid behavior and provides useful information for enhancing heat transfer in a variety of engineering applications.

Chemical reaction on MHD convective flow of second-grade fluid through a vertical porous channel with non-uniform wall temperature

We have examined the impacts of the suction and/ or injection on the magnetohydrodynamic convection fluctuating flow of secondary order fluids through the absorbent medium into a perpendicular channel by non-uniformed walls’ temperature. The fluid is subjected to a transversal magnetic domain as well as the velocity slips near the left side plate. The systematic resolutions of the non-dimensional equations were found and the impacts of the governed variables on velocity, temperatures, concentration profiles, skin frictions, rates of temperature and mass transports were explored. It is motivating to note that skin friction enhances channel plates as permeability enlarges. The magnitude of the velocity retards with an enhancement in Hartmann number and enlarges with an increase in the permeability parameter.

Aerothermal performances of a rotating turbine disk cavity with non-uniform wall temperature profiles in an aero-engine

Aerothermal performances of a rotating turbine disk cavity with non-uniform wall temperature profiles in an aero-engine

A designed wall roughness approach to improve turbulent heat transfer to supercritical CO2 flowing in horizontal tubes

Supercritical flow through a horizontal pipe leads to a non-uniform peripheral wall temperature distribution even when the wall heat flux is kept constant and uniform. This is attributed to lower heat transfer coefficient at the top section where the denser fluid tends to sink. Hence, to obtain a uniform wall temperature, a designed wall roughness is devised. Uniform sand-grain roughness is employed to only partly cover the top half of the pipe wall. Numerical simulations were conducted using the SST k−ω turbulence model. The simulation results indicate that our proposed design can lead to a more uniform heat transfer distribution over the wall periphery compared with the smooth pipe. An extreme case was also considered where the inner wall was completely covered with roughness elements. While heat transfer augmentation was observed for this case, the excess pressure drop was prohibitively higher compared with a pipe with designed wall roughness.

A regularized diffuse domain-lattice Boltzmann model for heat transfer in complex geometries with temperature Dirichlet boundary condition

In this paper, a regularized diffuse domain-lattice Boltzmann model for heat transfer with the Dirichlet boundary condition in complex geometries is proposed. In this model the complex computational domain is embedded into a larger and regular domain. The level set function is used to capture the complex interface. A diffuse domain formulation for the temperature field is established on the larger domain. To remove the ill-conditioned property of the original diffuse domain equation, we develop a new regularization technique to tackle this issue. We also give a multiple-relaxation-time lattice Boltzmann model to solve the regularized diffuse domain equation. The convergence rate of present model is investigated by the steady state and time-decay heat conduction problems. Then the natural convection problems in a square enclosure with a heated circular cylinder and in a horizontal concentric annulus are simulated. Both the uniform and non-uniform wall temperature cases are considered. Moreover, an alternative method to calculate the average Nusselt number on the complex boundary is employed. The numerical results show a good agreement with the previous solutions in previous literatures. The effect of non-uniform wall temperature on the heat transfer rate is also investigated. At last, the forced convection across the staggered tube banks is also simulated to verify the potential capability of the present model in solving practical heat transfer problem with the complex geometries.

Analysis of Hydro Magnetic Flow of Visco Elastic Fluid in a Vertical Channel

In the present paper we have Analysis of hydro magnetic flow of visco elastic fluid. Hydro magnetic visco elastic fluid flow between two horizontal infinite parallel porous plates with time varying sinusoidal pressure gradient and magnetic field has been discussed in the present study. The visco elasticity in an effective approach to modeling the dissipative mechanism. Some interesting observations are low-frequency oscillating pressure gradient Prevents back flow, significant reduction in skin frictions is observed by embedding the channel in porous medium and magnetic field and elasticity decelerate the fluid flow and also in this paper a Theoretical Analysis is carried out of study the visco elastic effects on hydro magnetic heat & mass transfer in a vertical channel. The two vertical plates are in porous medium and non-uniform wall temperatures. A magnetic field of uniform strength is applied in the direction perpendicular to the plates. The visco elastic fluid flow is characterized by second order fluid. The effects of different flow parameters on skin friction are analyzed and illustrated graphically.

Delineating impacts of non-uniform wall temperature and concentration on time-dependent radiation-convection of Casson fluid under magnetic field and chemical reaction

Purpose In various kinds of materials processes, heat and mass transfer control in nuclear phenomena, constructing buildings, turbines and electronic circuits, etc., there are numerous problems that cannot be enlightened by uniform wall temperature. To explore such physical phenomena researchers incorporate non-uniform or ramped temperature conditions at the boundary, the purpose of this paper is to achieve the closed-form solution of a time-dependent magnetohydrodynamic (MHD) boundary layer flow with heat and mass transfer of an electrically conducting non-Newtonian Casson fluid toward an infinite vertical plate subject to the ramped temperature and concentration (RTC). The consequences of chemical reaction in the mass equation and thermal radiation in the energy equation are encompassed in this analysis. The flow regime manifests with pertinent physical impacts of the magnetic field, thermal radiation, chemical reaction and heat generation/absorption. A first-order chemical reaction that is proportional to the concentration itself directly is assumed. The Rosseland approximation is adopted to describe the radiative heat flux in the energy equation. Design/methodology/approach The problem is formulated in terms of partial differential equations with the appropriate physical initial and boundary conditions. To make the governing equations dimensionless, some suitable non-dimensional variables are introduced. The resulting non-dimensional equations are solved analytically by applying the Laplace transform method. The mathematical expressions for skin friction, Nusselt number and Sherwood number are calculated and expressed in closed form. Impacts of various associated physical parameters on the pertinent flow quantities, namely, velocity, temperature and concentration profiles, skin friction, Nusselt number and Sherwood number, are demonstrated and analyzed via graphs and tables. Findings Graphical analysis reveals that the boundary layer flow and heat and mass transfer attributes are significantly varied for the embedded physical parameters in the case of constant temperature and concentration (CTC) as compared to RTC. It is worthy to note that the fluid velocity is high with CTC and lower for RTC. Also, the fluid velocity declines with the augmentation of the magnetic parameter. Moreover, growth in thermal radiation leads to a declination in the temperature profile. Practical implications The proposed model has relevance in numerous engineering and technical procedures including industries related to polymers, area of chemical productions, nuclear energy, electronics and aerodynamics. Encouraged by such applications, the present work is undertaken. Originality/value Literature review unveils that sundry studies have been carried out in the presence of uniform wall temperature. Few studies have been conducted by considering non-uniform or ramped wall temperature and concentration. The authors are focused on an analytical investigation of an unsteady MHD boundary layer flow with heat and mass transfer of non-Newtonian Casson fluid past a moving plate subject to the RTC at the plate. Based on the authors’ knowledge, the present study has, so far, not appeared in scientific communications. Obtained analytical solutions are verified by considering particular cases of the published works.

Experimental Study and Mitigation of Pressure Drop Oscillation Using Active Control

Abstract Flow boiling in microchannel evaporators is widely recognized and promising for its compact structure, lower coolant usage, high heat transfer coefficient, ability to provide higher heat fluxes, and better temperature uniformity than single-phase liquid cooling. However, critical heat flux (CHF), local dry-outs, and flow instabilities can be significant roadblocks for practical implementation. Flow instabilities, like pressure drop oscillation, could lead to nonuniform wall temperature distribution, flow reversal, and local dryout, which can be detrimental to system performance. We conducted an experimental study of a vapor compression cycle incorporating a microchannel evaporator to investigate the role of evaporator design and various system parameters on the overall performance. These parameters include the expansion valve setting, the accumulator heat load, and the evaporator heat load. While the evaporator design, the testbed, and system parameters affect the system response in unique ways, flow instability can be explained based on the overall pressure drop occurring in the system and how it varies as a function of these factors. Based on the understanding gained from this experimental study, a dynamic control strategy was developed to stabilize the system facing transient heat loads. The system can successfully address transient evaporator heat loads with feedforward control, which would otherwise lead to pressure drop oscillation. We believe this study can be helpful in further development of active control techniques to achieve multiple objectives of maintaining fixed evaporator temperature, allowing higher cooling rates, avoiding CHF, and suppressing flow instabilities, even in the presence of transient heat loads.

Irreversibilities in natural convection inside a right-angled trapezoidal cavity with sinusoidal wall temperature

Analysis on natural convective heat transfer in different engineering systems allows optimization of the technical apparatus. For this purpose, numerical simulation of the fluid flow and heat transport within the system is combined with study of entropy generation. The latter is very important considering the Gouy–Stodola theorem of thermodynamics. The present research deals with the mathematical modeling of thermal convection and entropy generation in a right-angled trapezoidal cavity under the influence of sinusoidal vertical wall temperature distribution. Control Oberbeck–Galerkin finite element technique has solved Boussinesq equations formulated using the non-dimensional primitive variables. Analyses of flow structures, thermal and entropy generation patterns for different values of the Rayleigh number, and parameters of non-uniform wall temperature were performed. It was found that a rise in the sinusoidal wall temperature amplitude increases the average Nusselt and Bejan numbers and average entropy generation. Moreover, growth in the non-uniform wall temperature wave number decreases the energy transport strength and Bejan number.

Transformation Magnetohydrodynamics in Presence of a Channel Filled with Porous Medium and Heat Transfer of Non-Newtonian Fluid by Using Lie Group Transformations

In this paper, the numerical results are presented by using Lie group transformations, to be more efficient and sophisticated. To solve various fluid dynamic problems numerically, we present the numerical results in a field of velocity and distribution of temperature for different parameters regarding the problem of radiative heat, a magnetohydrodynamics, and non-Newtonian viscoelasticity for the unstable flow of optically thin fluid inside a channel filled with nonuniform wall temperature and saturated porous medium, including Hartmann number, porous medium and frequency parameter, and radiation parameter, with a comparison of the corresponding flow problems for a Newtonian fluid. Moreover, the effects of the pertinent parameters on the friction coefficient of skin and local Nusselt number were discussed numerically and also illustrate that graphically.

Effects of micro-combustor geometry and size on the heat transfer and combustion characteristics of premixed hydrogen/air flames

major challenge in the development of micro-combustors is flame instabilities—resulting in a non-uniform wall temperature distribution and lower combustion efficiency. To overcome these issues, this research investigates the combustion characteristics of premixed hydrogen/air mixture in a micro-combustor with a cavity, bluff body, rib with bluff body and rib configurations. A detailed chemical reaction mechanism is also developed which consists of 13 species and 19 reactions. The obtained results are validated with published experimental findings. Having the model validated, a parametric study has been conducted to examine the effect of thermal conductivity coefficient, equivalent ratio, aspect ratio and geometrical configurations on heat transfer and combustion characteristics of hydrogen/air flames. It is demonstrated that increasing the thermal conductivity coefficient improves the preheating of the fresh mixture at upstream. However, this causes more heat loss from the outer walls to the surroundings. Moreover, increasing the equivalence ratio of a mixture reduces the negative effects arising from the heat losses on combustion stability. At higher inlet velocities, the location of a maximum temperature shifts towards downstream, which reduces the flame and average wall temperature. Among these configurations, a micro-combustor with bluff body is a more promising option to improve the flame stability and combustion efficiency.

Impacts of the periodic wall conditions on the hydromagnetic convective flow of viscoelastic fluid through a vertical channel with Hall current and induced magnetic field

AbstractIn this study, a mathematical analysis is presented for the hydromagnetic convective flow of an incompressible, chemically reacting, and electrically and thermally conducting viscoelastic fluid through a vertical channel bounded by the porous regime under the action of an applied magnetic field with Hall current and induced magnetic field effects. The left wall of the channel is considered to be nonmagnetic, whereas the right wall of the channel is periodically magnetized. The flow within the channel is induced due to the nonuniform wall temperature and concentration, periodic pressure gradient, and periodic movement of the right wall. The method of separation of variable is used to convert the flow governing coupled partial differential equations into the ordinary differential equations that are solved analytically, and the solution for fluid velocity, induced magnetic field, temperature, and concentration is presented in a closed form. Numerical computation has been performed to demonstrate the impact of various system parameters on the fluid flow behavior. It is observed that oscillations increase the primary flow and primary induced magnetic field. Buoyancy forces have a tendency to lessen the secondary induced magnetic field. Furthermore, it is examined that magnetic diffusivity increases the primary flow, whereas it decreases the secondary flow and primary induced magnetic field.

Unsteady MHD oscillatory visco-elastic fluid flow through an inclined channel in presence of chemical reaction with Soret and Dufour effects

The purpose of present paper is to analyze the Soret and Dufour effects on an unsteady magneto hydrodynamic oscillatory flow of radiative, visco-elastic fluid through an inclined channel filled with saturated porous medium with non-uniform wall temperature in presence of first-order chemical reaction. The governing dimensionless momentum equation coupled with the energy and mass diffusion equations are solved analytically. The expressions for velocity, temperature, concentration, skin friction, Nusselt number and Sherwood number are obtained and are analyzed graphically for various values of the dimensionless flow parameters.

Unconditional finite amplitude stability of a fluid in a mechanically isolated vessel with spatially non-uniform wall temperature

We investigate finite amplitude stability of spatially inhomogeneous steady state of an incompressible viscoelastic fluid which occupies a mechanically isolated vessel with walls kept at spatially non-uniform temperature. For a wide class of incompressible viscoelastic models including the Oldroyd-B model, the Giesekus model, the FENE-P model, the Johnson--Segalman model, and the Phan--Thien--Tanner model we prove that the steady state is stable subject to any finite perturbation.

Unconditional finite amplitude stability of a viscoelastic fluid in a mechanically isolated vessel with spatially non-uniform wall temperature

We investigate finite amplitude stability of spatially inhomogeneous steady state of an incompressible viscoelastic fluid which occupies a mechanically isolated vessel with walls kept at spatially non-uniform temperature. For a wide class of incompressible viscoelastic models including the Oldroyd-B model, the Giesekus model, the FENE-P model, the Johnson–Segalman model, and the Phan–Thien–Tanner model we prove that the steady state is stable subject to any finite perturbation.

Heat and Mass Transfer on Magnetohydrodynamic Convective Flow of Second Grade Fluid Through a Vertical Porous Channel with Non-Uniform Wall Temperature

We analyze the impact of suction/injection on MHD convective oscillatory flow of second grade fluid through a Porous Medium in a vertical channel with non-uniform wall temperature. The liquid is occupied with a transverse Magnetic field and the velocity slip at the left plate is taken into consideration. The precise solutions of the dimensionless equations are obtained and the effects of the flow parameters on velocity, temperature and concentration profiles, skin friction and rates of heat and mass transfer are discussed. It is fascinating to take note of that skin friction increases on together channel plates as permeability increases.

Comparison of urban airflow between solar-induced thermal wall and uniform wall temperature boundary conditions by coupling CitySim and CFD

Previous studies investigated the urban wind airflow under the isothermal condition, solar-induced thermal wall and uniform wall temperature boundary conditions. The differences in spatially-averaged airflow properties among those three boundary conditions have not been investigated yet. This study investigated different scenarios to identify: (1) whether the solar-induced non-uniform wall temperatures could be replaced by the uniform wall temperatures (average of all wall temperatures from solar-induced scenario) in the analysis of urban wind airflow and (2) the wind conditions that the wall thermal boundary conditions might be neglected. In this study, urban energy model, CitySim, was employed to calculate the solar-induced walls’ temperatures of an idealized 5 × 5 building array, which were set as thermal boundary conditions in computational fluid dynamic (CFD) simulations. The simulation results were compared to those obtained from cases with uniform wall temperatures and isothermal conditions. The results showed that excluding wall thermal boundary conditions underestimated the spatially-averaged flow properties when reference wind speed (Uref) was lower than 2.0 m s−1 under the designed scenarios and noticeable difference of airflow pattern was observed between isothermal and non-isothermal conditions. The solar-induced wall temperature condition might be replaced by uniform wall temperature condition with Uref of 3.0 m s−1 under parallel wind condition, and with Uref of 5.0 m s−1 under oblique wind conditions. Meanwhile, the airflow structures of the street canyons inside the three-dimensional building array differed significantly from the previous results obtained from isolated street canyon model.