Wall Temperature Research Articles

Effect of slip boundary conditions on unsteady pulsatile nanofluid flow through a sinusoidal channel: an analytical study

A novel analysis of the pulsatile nano-blood flow through a sinusoidal wavy channel, emphasizing the significance of diverse influences in the modelling, is investigated in this paper. This study examines the collective effects of slip boundary conditions, magnetic field, porosity, channel waviness, nanoparticle concentration, and heat source on nano-blood flow in a two-dimensional wavy channel. In contrast to prior research that assumed a constant pulsatile pressure gradient during channel waviness, this innovative study introduces a variable pressure gradient that significantly influences several associated parameters. The mathematical model characterising nano-blood flow in a horizontally wavy channel is solved using the perturbation technique. Analytical solutions for fundamental variables such as stream function, velocity, wall shear stress, pressure gradient, and temperature are visually depicted across different physical parameter values. The findings obtained for various parameter values in the given problem demonstrate a significant influence of the amplitude ratio parameter of channel waviness, Hartmann number of the magnetic field, permeability parameter of the porous medium, Knudsen number due to the slip boundary, volume fraction of nanoparticles, radiation parameter, Prandtl number, and heat source parameters on the flow dynamics. The simulations provide valuable insights into the decrease in velocity with increasing magnetic field and its increase with increasing permeability and slip parameters. Additionally, the temperature increases with increasing nanoparticle volume fraction and radiation parameter, while it decreases with increasing Prandtl number.

Optical Study on the Effects of Wall and Intake Temperatures on Ammonia Combustion and Emissions

Optical Study on the Effects of Wall and Intake Temperatures on Ammonia Combustion and Emissions

Microscopic insights on liquid–vapor phase change phenomena of R1336mzz(Z) nanofilm through molecular dynamics simulations

The ever-growing advances in electronics pose a critical cooling challenge. Two-phase heat transfer technology, with effective heat removal and no extra energy consumption, is emerging as a potential solution for sustainable thermal management. However, questions regarding phase change mechanisms are seemingly not addressed yet due to the limitation in time and space scales. In this work, a series of molecular dynamics simulations are performed where the R1336mzz(Z) liquid film rests over a smooth copper substrate. The molecular system starts with a nanofilm thickness of 4–10 nm and then is subjected to non-equilibrium heating (350, 400, 450, and 500 K) to initiate liquid-to-vapor phase change. Depending on the impacts of film thickness and wall temperature, four distinctive phase change scenarios are identified, i.e., diffusive evaporation, pseudo-diffusive evaporation, explosive boiling, and stable explosive boiling. The characteristics of these phase change modes are explicated from the perspectives of molecular motion, vaporization rate, temperature evolution, heat flux, and interfacial resistance. The phase change mode has a strong dependence on two aspects: improving heat transfer to breakup potential barrier and slowing energy dissipation to support formation of bubble embryos. The insight may deliver better understandings of phase change mechanism and applications of liquid refrigerants.

Design, Simulation and Optimization of a Novel Transpired Tubular Solar Air Heater

In this paper, a novel tubular solar air heater is introduced. In this air heater, the hot boundary layer is drawn into the absorber tube and can provide thermal energy at moderate temperatures. Several different cases were simulated and a correlation was proposed to predict the collector’s effectiveness as a function Rayleigh number and Reynolds number. An equation was derived to find the effectiveness of this collector. Finally, a real case was studied with non-uniform solar flux distribution, as well as radiation heat loss. Good agreement was found between the results and those derived by the proposed analytical method. For different suction values, the first-law and the second-law efficiencies were calculated. Based on the exergy analysis, exergy destruction in absorption is the dominant factor that is unavoidable in low-temperature collectors. It was shown that there is an optimum suction value at which the second-law efficiency is maximized. At the optimum point, temperature rise can reach 54 K, which is hardly possible with a flat plate collector. Based on the exergy analysis, the relation between tube wall temperature and air outlet temperature in their dimensionless forms at the optimum working condition was derived, and it was shown that effectiveness at the optimum working condition is around 0.5. This means that the air temperature rise shall be half of the temperature difference between collector wall and the ambient temperatures. A high outlet temperature besides the low cost of construction and maintenance are the main advantages of this air heater. With such a high temperature rise, this type of collector can increase the use of solar energy in domestic applications.

A Comparative Model of Thermal Performances Between a Modern and Classic Habitat for A Hot and Humid Tropical Climate: Case of Guinea

The objective of this research is to carry out a comparative study between a classic habitat and a modern habitat in order to understand which of the two is more efficient for a humid tropical area. To do this, a digital modeling of the heat transfer phenomena in the different components of the habitat was highlighted for a habitat made of stabilized earth bricks, a roof made of transparent tiles, false ceilings made of rectangular panels containing phase change material, concrete floor and a classic habitat made from local materials is presented. To show the reliability of this study, we validated our habitat model integrating phase change material through an experimental study carried out by researchers B. ZIVKOVIC and I. FUJII. A study on the temperature distributions at the level of certain components of the habitat done, and the influence of the variation of certain parameters was highlighted to know the thermal behavior of the two habitats. Furthermore, a comparative study was performed between the interior and exterior temperatures of the south walls of habitats incorporating PCM and without PCM, which are the components most exposed to solar radiation whose maximum values are respectively 33°C and 34, 5°C and 33.15°C against 47°C. Keywords: Performances, thermal, bioclimatic, PCM, Tropical climate, Guinea.

Thermal and Entropic analyses of free convection of nanofluid in a partially heterogeneous porous chamber using the two-phase mixture model

Entropy generation and convection heat transfer in a partially porous chamber with different side wall temperatures using CuO–H2O have been investigated. The importance of this issue is wide application of the results in solar collectors, thermal extrusion systems, heat exchangers, bio-medicine, nuclear waste disposal, etc. The innovation of the present work is related to the investigation of fluid and heat fields and entropy generation by using a matrix with subordination of porosity to the vertical axis and permeability, thermal conductivity, and viscosity with subordination of porosity. To obtain accurate results, the two-phase mixture model was used, and thermal conductivity and viscosity of nanofluid were simulated by experimental models by temperature and volume fraction dependence. Governing equations are solved by the FVM. The main findings indicate that the best and worst optimization factor will occur in the porous matrix ε = ε(y2) and ε = -ε(y2), respectively, which is 113 % and 86 % of NH of the homogeneous matrix, respectively. Also increasing the filling of the cavity, highly improves NH, so that the NH will reach from 1.19 to 1.82 with the increase of S from 0.25 to 1.

Heat transfer error analysis of high-temperature wall temperature measurement using thermocouple

Thermocouples are widely employed as the temperature measuring element for an aero-engine. However, due to the unique characteristics of the aero-engine test environment, there exist significant temperature measurement errors of the thermocouple. The steady-state heat transfer error of the thermocouple can be attributed to convection heat transfer error, wire heat transfer error, and radiation heat transfer error. The flow and heat transfer process of wall temperature measurements using armored K-type thermocouples were simulated numerically by Fluent software at the Mach number of 0.1–0.5 and the temperature of 700∼1200 K. The results indicate that convective error accounts for more than 55 % of the total transfer error. To reduce excessive temperature measurement errors, a novel airflow stagnation cover structure around the thermocouple was presented. When the diameter of the stagnation cover is 2 mm, the error of the thermocouple is reduced to less than 1 %.

Numerical investigation of inserting conical ring into the micro-combustor with premixed hydrogen/air to enhance the heat transfer performance for the micro-thermophotovoltaic system

The low thermal performance of conventional micro-combustor (CMC) is one of the major obstacles hindering the development of micro-thermophotovoltaic (MTPV) system. In the present research, micro-combustor with conical ring (MCCR) is proposed to enhance the overall performance. A comparative study of the temperature field, pressure field, velocity field and combustion efficiency of the CMC and MCCR is conducted. The numerical results show that when the inlet velocity is 8 m/s, compared with the CMC, the mean outer wall temperature, combustion efficiency and pressure loss of the MCCR-6 increased by 32.3 K (2.6%), 0.05% and 718.6 Pa (273.4%), respectively, while the standard deviation of outer wall temperature decreased by 31.4 K (49.5%). Additionally, the MC made using SiC provides a better overall performance, the conical ring (CR) is installed at 6 mm from the step with better overall performance.

Experimental study of boiling heat transfer characteristics of multi-elbow cooling system under one-sided heating with long section for fusion divertor cooling

In this study, heat transfer experiments were conducted under one-sided heating conditions using water as the coolant and an electron gun as the heat source. To evaluate the boiling heat transfer characteristics of the swirling flow formed by a multi-elbow piping, heat flux by the electron gun is deposited on the region of 60 mm-section in the flow direction corresponding to 0.5 D (inner diameter of the flow channel) to 6.5 D from the elbow outlet, and then temperature distributions in the flow, circumferential and radial directions were measured. The experimental results showed that the wall temperature cooled by the swirling flow generated downstream of the multi-elbow was lower than that obtained in the case of turbulent flow in the straight pipe when irradiated with the same value of heat flux. Thereby, the heat fluxes at the boiling start and DNB points obtained in the multi-elbow case were higher than those in the straight pipe case. These results imply the superior cooling performance of multi-elbow system.

Heat transfer enhancement in a triple-layered turbine blade internal cooling channel

An effective measure of enhancing the coupled convection and radiation in the turbine blade internal cooling channel is developed. A triple-layered channel is presented for the first time to further improve the flow organization to suppress the negative effect of secondary flows induced by rotation as well as extend cold surfaces for radiation. Explorations of flow and coupled heat transfer are executed numerically for various channel models. The results manifest that, heat transfer is markedly improved with the triple-layered channel. The highest comprehensive thermal performance is achieved when employing the triple-layered channel with smooth surfaces and ribs (Model 3SW), which can increase the total Nusselt number ratio by 105.78 % and performance evaluation criterion by 18.75 %, comparing with the existing Model 1 N. Besides, the average and maximum wall temperature drop by 54.26 K and 293.02 K, respectively. In addition, the triple-layered channel is demonstrated to have the ability to reduce rotational effects on heat transfer uniformity. The heat transfer difference between the leading and trailing walls induced by the Coriolis force is restricted. It also has the self-adaptive ability to increase the degree of heat transfer enhancement when thermal load increases.

Experimental study on heat transfer characteristics of dispersed flow during strong transient reflood process

The dispersed heat transfer correlations obtained in the quasi-steady-state heating experiment may have some limitations in the strong transient reflood process. Therefore, this article will conduct a visualization experiment on single-rod channel reflood cooling under different working conditions. By combining the transient hydrodynamic characteristics of liquid droplets and the wall temperature transient curve, the boiling heat transfer characteristics of the dispersed flow stage during the reflood process are studied. Through comparative analysis, this article found that the traditional post-CHF (Critical Heat Flux) heat transfer correlations are difficult to characterize the impact of droplet number and size changes. This article is based on the form of a divided path heat transfer mechanism model. In response to the strong transient characteristics of dispersed flow during the reflood process in a restricted channel, the single-phase steam heat transfer model and the steam-droplet interfacial heat transfer model have been modified, greatly improving the prediction accuracy. The RMS of the predicted heat transfer coefficient decreased from 0.68 in the original model to 0.24 in the new model.

Investigation of local Nusselt number in a rarefied microchannel gas flow using Direct Simulation Monte Carlo method

Heat transfer in microscale and rarefied gaseous flows has been investigated for high Knudsen number (Kn) flows using the Direct Simulation Monte Carlo (DSMC) method. A wide range of Kn [0.05 – 5], pressure ratio [1.2 – 4] and wall temperature [323 K – 723 K] is studied. The heat flux and local Nusselt number (Nu) are found to exhibit three distinct sections along the length of the microchannel – heat transfer-dominated section, constant heat transfer section and exit section. The defining roles of different effects such as non-equilibrium conditions at the wall, viscous dissipation, axial conduction, pressure work, and shear work have been considered implicitly in the DSMC method. These parameters are seen to influence the heat transfer differently at lower and higher Kn. These results help to quantify the fraction of total heat exchange and reveals the complex interplay of various terms of the energy equation in gas flow in microchannels.

Heat transfer characteristics of horizontal flow boiling with uniform and non-uniform circumferential heat flux

This study assesses effects of the non-uniform circumferential heat flux on horizontal flow boiling of R141b in a stainless-steel tube at ambient system pressure with an inner diameter of 10.92 mm. The main motivation for this are application in Concentrated Solar Power using the Direct Steam Generation process inside horizontal absorber tubes. Experiments were conducted for two mass velocities (50 and 150 kg m−2s−1) and two mean heat fluxes (3.4 and 11.0 kW m−2), with three different heating conditions (uniform, top-concentrated, and bottom-concentrated). The differences in the Onset of Nucleate Boiling between uniform and bottom-concentrated heating are assessed, and the transient behavior of the wall temperatures is investigated. It is found that the heating condition significantly affects the mean heat transfer coefficient in the case of non-annular flow regimes but practically not in annular flow. The circumferential wall temperature gradient is with uniform heating highest for non-annular flow regimes. But with non-uniform heating the circumferential wall temperature gradient is highest for annular flow. The use of empirical correlations for the heat transfer coefficients are evaluated.

A conjugate heat transfer investigation of outer-wall cooling performance in double-wall vane pressure side

Double wall cooling has significant potential for high-pressure turbine vane cooling technology. Geometric parameters of the double-wall vane play a crucial role in determining the cooling performance. This paper presents a double-wall cooling model with five outer wall thicknesses and three injection angles, using the curvature of the actual pressure side of the engine as a reference. The overall cooling effectiveness is investigated over the range of blowing ratio from 0.5 to 2.5. Results indicate that, at injection angles of 20° and 30°, an outer-wall thickness of 1.5D f maximizes the combined benefits of film cooling and thermal conductivity in solid domains. At an injection angle of 40°, the blowing ratio becomes a determining factor in selecting the optimal outer wall thickness. Additionally, a reasonable arrangement of pin fins promotes a more uniform outer wall temperature distribution. A thinner outer wall at higher blowing ratios provides optimum overall cooling effectiveness.

Numerical study on the local aggregation of helium bubbles in liquid lithium and its thermal analysis

Abstract Liquid lithium is widely regarded as an optimal cooling medium for space nuclear reactors due to its exceptional heat transfer properties and low density. However, the helium bubbles generated by liquid lithium under space irradiation pose significant hazards to the safe and stable operation of nuclear reactions. In this study, the localized accumulation of helium bubbles in liquid lithium is investigated using a two-phase flow turbulence model. The effects of helium bubble distribution and inlet velocities on various parameters in the pipeline are focused on. A non-isothermal model for bubble flow is developed to examine the influence of gas-liquid mixture concentrations on overall heat transfer performance under low concentration conditions. Agglomerated bubbles with radii between 5 μm and 150 μm are classified into three categories based on local concentrations: circular (≤20.37%), irregular elongated (up to 30.44%), and banded (up to 36.31%). Interconnected banded bubbles can be up to 8 times larger than irregularly elongated ones, impacting physical properties and wall temperature disturbance in the pipeline. Elevated inlet velocity initiates the occurrence of bubble impact and fragmentation. However, high flow rates near the wall do not diminish wall temperature disturbance. Mixed flows with bubbles scales <15 μm show no significant impact on overall heat transfer up to 1% concentration. This study reveals the effects of bubble number and distribution, providing insights for manipulating bubble structure and guiding localized and comprehensive thermal analyses.

Effects of turbulent Prandtl number on supercritical carbon dioxide turbulent flow with high heat flux in vertical round tube

Supercritical carbon dioxide (sCO2) is a promising working medium for coal-fired power plants, high-temperature solar power systems, nuclear reactor systems, and fuel cells owing to its high efficiency and inertness. In these cases, the sCO2 is in a round tube under high temperature and pressure, far from the pseudo-critical point. Here, the thermophysical properties of sCO2 changes, resulting in large prediction errors. In this study, a conjugate heat transfer model with the k-ε turbulent model was created to analyze the tube performance of the sCO2, and the effects of turbulent Prandtl number (Prt) on the sCO2 turbulent flow with high heat flux were examined through experimental and numerical investigations. Prt had a considerable effect on the performance of the sCO2 turbulent flow far from the pseudo-critical point. The mean relative errors of the inner wall temperature and convective heat transfer coefficient were reduced by approximately 13% and 27%, respectively, when Prt was decreased from 1 to 0.55. For the sCO2 turbulent flow with a high heat flux, constant Prt values of 0.55–0.6 were applicable. The findings afford key insights for the development of sCO2 turbulent flow with high heat flux.

Optimization study on acid condensation and anti-corrosion performance of 3-D finned tube heat exchangers

Optimization study on acid condensation and anti-corrosion performance of 3-D finned tube heat exchangers

Numerical simulation on in-vessel molten corium behavior with external vessel cooling using smoothed particle hydrodynamics

The in-vessel retention through external reactor vessel cooling (IVR-ERVC) strategy is a key management strategy for early termination of a nuclear severe accident that can threaten the integrity of the reactor vessel. To simulate the physical phenomena of the molten corium, the smoothed particle hydrodynamic (SPH) method is utilized in this study. The SPH method is a Lagrangian computational fluid dynamic (CFD) method that can simulate multi-fluid stratification, turbulence, natural circulation, radiative heat transfer, thermal ablation, and crust formation. To address the external vessel cooling, it is coupled with a conventional 1-D nuclear system analysis method. The 1-D system analysis code can calculate the two-phase natural circulation of cooling water and the convective heat transfer on the external reactor vessel wall. These two simulation codes exchange the temperature and heat flux of the reactor vessel outer wall. This study numerically simulated the IVR-ERVC strategy for a Korean high-power reactor and compared it with the traditional lumped parameter method (LPM). Unlike LPM, this study provides localized detailed data about the thermal hydraulic behavior of molten corium and visualization of phenomena in the IVR-ERVC strategy. This enhances our understanding of the phenomena in IVR-ERVC strategy and introduces new perspectives.

In-flight anti-icing simulation of electrothermal ice protection systems with inhomogeneous thermal boundary condition

Conventional anti-icing computational solvers calculate the convective heat transfer coefficient using a homogeneous thermal boundary condition, assuming a constant temperature across the surface. However, this approach can lead to inaccuracies in regions with significant temperature variations. To address this limitation, the present study proposes a novel approach that utilizes an inhomogeneous thermal boundary condition, which updates the convective heat transfer coefficient based on the transient wall temperature distribution. We developed a unified finite volume framework that efficiently integrates various in-house solvers, including a compressible Navier-Stokes-Fourier (NSF) airflow solver, a Eulerian droplet impingement solver, a PDE-based ice solver, and a multilayer heat conduction solver. Thermal interaction between the different solvers was modeled using the conjugate heat transfer method. The effects of updating airflow on anti-icing results were investigated by comparing the results obtained by coupled and decoupled solvers. While the decoupled solver computes airflow once and remains unchanged, the coupled solver updates the airflow during the anti-icing simulation. Our findings show that the decoupled solver has the highest deviations from the coupled solver in evaporative anti-icing regimes, where dry regions form within the protection limits. Using coupled solvers in designing ice protection systems in evaporative regimes improves temperature prediction accuracy, enabling designers to reduce safety factors and save energy.

Surface recombination in Pyrex in oxygen DC glow discharges: mesoscopic modelling and comparison with experiments

Surface recombination in an oxygen DC glow discharge in a Pyrex (borosilicate glass) tube is studied via mesoscopic modelling and comparison with measurements of recombination probability. A total of 106 experimental conditions are assessed, with discharge current varying between 10 and 40 mA, pressure values ranging between 0.75 and 10 Torr, and fixed outer wall temperatures ( Tw ) of −20, 5, 25 and 50∘C . The model includes O+O and O+O2 surface recombination reactions and a Tw dependent desorption frequency. The model is validated for all the 106 studied conditions and intends to have predictive capabilities. The analysis of the simulation results highlights that for Tw=−20∘C and Tw=5∘C the dominant recombination mechanisms involve physisorbed oxygen atoms ( OF ) in Langmuir–Hinshelwood (L-H) recombination OF+OF and in Eley–Rideal (E-R) recombination O2+OF , while for Tw=25∘C and Tw=50∘C processes involving chemisorbed oxygen atoms ( OS ) in E-R O+OS and L-H OF+OS also play a relevant role. A discussion is taken on the relevant recombination mechanisms and on ozone wall production, with relevance for higher pressure regimes.