Top Wall Heating Research Articles

Experimental investigation on identification of heat transfer deterioration and optimization of correlations for supercritical CO2

The existing experimental research on heat transfer deterioration of supercritical CO2 (S-CO2) in horizontal heated tubes and how to improve accuracy of heat transfer correlations by identifying this deterioration is still lacking. This paper analyzes the effects of inlet pressure, heat flux and mass flux on heat transfer across various states, with focus on precise heat flux intervals near state transitions. The results indicate that heat transfer at top wall and bottom wall can exhibit significant differences, necessitating independent analysis for accurate understanding and prediction. As heat flux increases during transition from liquid region to liquid-like region, heat transfer coefficient exhibits an upward trend. Both buoyancy and flow acceleration effects have significant impacts on local heat transfer near the pseudocritical region. An optimization method for heat transfer correlations has been proposed and validated. When an optimal value of 0.8 for heat transfer coefficient ratio is set by referencing Dittus-Boelter equation, newly introduced correlation predicts top wall heat transfer coefficient with 95.76 % of data points falling within a ±15 % error margin. The study identifies key factors that influence the deterioration of heat transfer, proposing correlations for identifying critical heat flux and practical measures to mitigate deterioration in industrial applications.

Partial magnetic field and segmental heating effects on hybrid nanofluidic convection in a tilted porous wavy cavity

The present study investigates the magneto-thermal convective process in an oriented porous wavy-walled geometry filled with Cu–Al2O3/water hybrid nanofluid, which is partially heated from the upper and lower walls under a partially active magnetic field. The sidewalls are cooled, while heating is carried out from the upper or lower walls or both. The flow-physics are greatly affected by the segmental heating and their positions, sidewall curvature, spatially active partial magnetic field, and all the flow-controlling variables. The transport equations are solved using the finite volume technique with SIMPLE and TDMA solver-based computing code. The hydro-thermal behavior of the thermal system is evaluated for several important variables, such as the Darcy-Rayleigh number, Hartmann number, Darcy number, concentration of hybrid nanofluid, partially active magnetic field strength, inclination and positions, geometry orientation, length of segmented heaters, and their position. It is revealed that the overall thermal transport phenomena can be precisely controlled by adjusting the position and dimension of segmented heaters, the active zone and direction of the imposed magnetizing field, and the cavity angle. Both the cavity angle and magnetic field angle of 90° enhance heat transfer. Top wall heating produces weak circulation. In the case of position study for segmental heating, the sidewall position is the best location for maximum heat transfer. The circulation strength and heat transfer are maximum for the bottom heated scenario, and the transfer enhancement goes up to 132.91%. In addition, an entropy generation analysis is conducted to understand the order of thermal, viscous, and magnetic effects on irreversibility production. The results demonstrate the effectiveness of the proposed approach in optimizing the system's performance.

Study of Rayleigh-Taylor instability in viscosity-stratified fluid layers

The Rayleigh-Taylor instability in viscosity varying fluid layers is studied. A heavier fluid layer is superimposed on a lighter fluid layer. Both the fluid layers are bounded between two parallel isothermal horizontal walls. Out of the two walls, one wall is isothermally heated, while the other wall is isothermally cooled. The Rayleigh-Taylor instability is studied for axisymmetric configuration. Viscosities of both the fluids are considered to be varying with temperature. The effect of viscosity ratio, temperature ratio, heating location, Prandtl number and Weber number on the instability is studied. When the viscosity of fluid layers vary with temperature, the configuration becomes more unstable compared to that for constant viscosity fluid layers. For varying viscosity fluid layers the spike undergoes large deformations. At lower viscosity ratios, mushroom shaped spike is formed with elongated skirt structure. Whereas, at higher viscosity ratios, spike with fluid column structure is formed. It is found that increasing viscosity ratio shows stabilizing effect. Increasing temperature ratio is found to show destabilizing effect with complex spike structure formation at high temperature ratios. In top wall heating configuration, formation of mushroom shaped spike and skirt of the spike occurs earlier in a thinner and elongated form compared to that of bottom wall heating configuration. Prandtl number showed insignificant effect on the instability. For the parameter values of the study, when Weber is lower than 55 the configuration is stable and does not undergo instability. Overall, surface tension has shown stabilizing effect on the instability.

Computational investigation of bounded domain with different orientations using CPCM

Abstract The present work deals with the composite phase change material (CPCM) of 98% paraffin wax and 2% copper nanoparticle, filled into the bounded domain. Effects of orientation (45°, 90°, 135° and 180°) with different wall heating conditions (base, left and top wall) are analyzed numerically to understand the flow patterns and interface morphology developed during melting/solidification processes. The melting/solidification mechanism exhibited non-uniform flow patterns and irregular morphology which are dependent on geometrical orientations and different wall heating conditions. The results revealed that the bounded domain with different orientations have significant effect on natural convection current formation. As the orientation changes, the heat transfer rate gets influenced significantly and convection currents amplifies. Top wall heating arrangement of 180° orientation shows competence in achieving better thermal performance.

Numerical study of turbulent flow in asymmetrically heated horizontal channel

Low Reynolds number turbulent flow of Helium in a horizontal channel subjected to asymmetric heating was studied using a low-Reynolds number turbulence model. The study was conducted for inlet Reynolds number ranging from 4000 to 10,000, non-dimensional wall heat flux ranging from 0.001 to 0.005, and ratio of top wall heat flux to bottom wall heat flux ranging from 0 to 1. The results from the study were used to obtain laminarization criterion as a function of inlet Reynolds number, non-dimensional wall heat flux, and ratio of top wall heat flux to bottom wall heat flux. The developed laminarization criteria which is based on the ratio of turbulence production to turbulence dissipation predicts a reduction in the Nusselt number as compared to the constant property turbulent flow results, ranging from as much as 45% reduction for inlet Reynolds number of 4000 to 22% reduction for inlet Reynolds number of 10,000.

Effects of subcooling and two-phase inlet on flow boiling heat transfer and critical heat flux in a horizontal channel with one-sided and double-sided heating

This study explores the influence of inlet subcooling and two-phase inlet on flow boiling heat transfer and critical heat flux in a horizontal 2.5-mm wide by 5-mm high rectangular channel for top wall heating, bottom wall heating and double-sided heating configurations using FC-72 as working fluid. High-speed video imaging is used to identify dominant interfacial characteristics for different combinations of inlet conditions and heating configurations. Three inlet conditions are compared: highly subcooled liquid, slightly subcooled liquid, and saturated two-phase mixture for mass velocities between 205.1 and 3211.6kg/m2s. Gravity is shown having a dominant influence on interfacial behavior at low mass velocities below 400kg/m2s, while inertia dwarfs gravity effects at high mass velocities around 1600kg/m2s. Overall, CHF increases monotonically with increasing inlet subcooling. CHF variation between the three heating configurations is large for low mass velocities and diminishes for high mass velocities. A dimensionless parameter, heat utility ratio, is shown to be an effective means for assessing the influence of subcooling on CHF.

Flow boiling and critical heat flux in horizontal channel with one-sided and double-sided heating

This study explores flow boiling of FC-72 along a 5-mm high by 2.5-mm wide rectangular channel that is fitted with top and bottom heating walls. By activating one wall at a time, the opposing influences of gravity are examined for inlet velocities from 0.11 to 2.02m/s. Results for top wall and bottom wall heating are then compared to those for double-sided heating. For top wall heating, high speed video imaging proves gravity effects are dominant at low velocities, accumulating vapor along the heated wall and resulting in low critical heat flux (CHF) values. For bottom wall heating, buoyancy aids in vapor removal and liquid replenishment of the heated wall, resulting in higher CHF values. Higher velocities result in fairly similar interfacial behavior for top wall and bottom wall heating, and double-sided heating exhibiting greater symmetry between interfacial behaviors along the opposite walls. Overall, CHF values for all three configurations converge to one another above 1.5m/s. This convergence is clearly the result of high inertia negating the influence of gravity. It is shown that interfacial instability theory provides an effective means for assessing the influence of velocity on CHF for top wall versus bottom wall heating. For top wall heating, a stable interface at low velocities causes vapor accumulation against the top wall resulting in very low CHF. Instability theory shows that top wall heating becomes unstable above 1.03m/s, allowing liquid contact with the wall and improved wall cooling. For bottom wall heating, the interface is always unstable and favorable for liquid contact. Instability theory also shows that inertia dwarfs gravity around 1.5m/s, where critical wavelengths for top wall and bottom wall heating converge. Convergence of the CHF values for top wall and bottom wall heating also occurred at a similar value.

Numerical study of combustion and convective heat transfer of a Mach 2.5 supersonic combustor

In this paper, characteristics of combustion and convective heat transfer of a supersonic combustor at two fuel/air equivalence ratios of 0.9 and 0.46 were numerically studied. The numerical method of Favre averaged Navier–Stokes simulation with SST k-ω turbulence model and a multiple-step reaction mechanism of ethylene is introduced. The inlet Mach number of the combustor is 2.5 and inlet total temperature is 1650K, corresponding to Mach 6 flight conditions. Ethylene is injected at two locations upstream of a flame-holding cavity. The numerical method was validated by comparing the present results of wall pressures and heat fluxes to experiments and theoretical analysis. It is found that, due to injection of fuel at the bottom wall, fuel/air mixing and combustion occurs mainly in the vicinity of the bottom wall. High non-symmetry in distributions of the bottom and the top wall heat fluxes is observed. Peaks of wall heat flux at different locations and at varied fuel/air equivalence ratios are identified, which are caused respectively by effect of cavity and by shock structure formed upstream of the injection points. It is also found that heat flux peaks are strongly related to the reaction step of CO→CO2, contributing to major heat releasing.

Phase Change in a Three-Dimensional Rectangular Cavity Under Electromagnetically Simulated Low Gravity: Top Wall Heating With an Unfixed Material

ABSTRACT In this article, melting of an unfixed phase-change material (PCM), gallium, in a simulated low-gravity environment via an electromagnetic field has been modeled numerically in a three-dimensional enclosure. Both transverse electric and magnetic fields are used to generate a Lorentz force, which opposes the influence of gravity. This Lorentz force acts to damp and/or counteract the convective flow induced by the floatation of the solid PCM within the melt, thereby simulating the low-gravity environment of outer space. The problem is formulated as one-domain by employing an enthalpy-based transformation of the energy equation, which allows for one set of governing equations to be solved. The governing equations are then discretized using a control-volume-based finite-difference scheme. The results show that key melting characteristics found under a true low-gravity environment can be simulated by both the application of a magnetic field alone or by the application of an electromagnetic filed. Each case of simulated low gravity has distinct advantages and disadvantages, such that the preferred method of simulation will depend on which characteristics are of more importance. The magnetic-only case allows for lower levels of gravity to be obtained. However, there is a significant distortion of the flow field and a resultant effect of the transient region of the solid velocity. The electromagnetic case allows for more accurate representation of the transient solid velocity, but there is a sacrifice in the level of low gravity achievable.

NUMERICAL SOLUTION OF MELTING PROCESSES FOR FIXED AND UNFIXED PHASE CHANGE MATERIAL IN THE PRESENCE OF MAGNETIC FIELD--SIMULATION OF LOW-GRAVITY ENVIRONMENT

Transport processes associated with melting of an electrically conducting phase change material (PCM), placed inside a rectangular enclosure, under a low-gravity environment, and in the presence of a magnetic field, is simulated numerically. Electromagnetic forces damp the natural convection as well as the flow induced by sedimentation and/or floatation, and thereby simulate the low-gravity environment of outer space. Computational experiments are conducted for both side-wall heating and top-wall heating under a horizontal magnetic field. The governing equations are discretized using a control-volume-based finite difference scheme. Numerical solutions are obtained for a true low-gravity environment as well as for the simulated low-gravity conditions that are a result of the presence of a horizontal magnetic field. The effects of magnetic field on the natural convection, solid phase floatation/sedimentation, liquid/solid interface location, solid melting rate, and the flow patterns are investigated. It is found that the melting under a low-gravity environment reasonably can be simulated on earth via applying a strong horizontal magnetic field. However, the flow patterns obtained for the true low-gravity environment are not similar to the corresponding cases solved for the simulated low gravity.