Important Role In Heat Transfer Research Articles

A new methodology to calculate interface thermal resistance between intumescent coating and steel substrate

Interface thermal resistance (ITR) plays a very important role in heat transfer and thermomechanical coupling response from intumescent coating to steel substrate in fire. However, the dynamic ITR between intumescent coating and steel substrate during burning is seldom reported. In this paper, a new methodology is presented to calculate dynamic ITR between intumescent coating and steel substrate under high incident heat flux based on the interface conservation equation. The new methodology focuses on measuring the temperature distribution and thermal conductivity of intumescent coating and steel substrate, with fewer variables. In this way, the calculation of ITR can be simplified. Firstly, the temperature variation function curves inside the intumescent coating and the steel substrate were fitted separately based on the temperature measurement data. Secondly, the interface temperatures (Tci,Tsi) were determined separately at the interface position by temperature variation function curves inside the intumescent coating or the steel substrate. Finally, the ITR calculation formula was deduced and the key parameters required for ITR calculation were obtained by cone calorimetry et al. An illustrative example was studied and validate the applicability of the new methodology.

Experimental and numerical investigation on the heat and mass transfer performance of tar rich coal in-situ pyrolysis

On ground experiments are conducted firstly to simulate the multi-physics underground pyrolysis of tar rich coal and find out the kinetic parameters of the reaction. And then multi-physics simulation is performed by coupling heat transfer, fluid flow and chemical reactions. Three pyrolysis models are compared and it’s found that the convective flow of heat carrier in coal seam plays an important role in heat transfer although the flow velocity is low. Then the effects of permeability, inlet mass flow rate on heat and mass transfer are investigated for two geometric models with homogeneous and inhomogeneous permeability. It’s found that increasing the permeability is beneficial for fast production of tar. For the homogeneous permeability model, the pyrolysis reaction can be completed in only 140 days when the permeability is 1000 md while for the non-homogeneous permeability model, increasing the inlet mass flow rate can improve the heat and mass transfer performance only in the early stage when the pyrolysis occurs mainly in the fractured zone with higher permeability. The pyrolysis reaction time when the inlet mass flow rate is 0.174 kg/s is 22 days shorter than that of 0.054 kg/s. But in the later stage the inlet mass flow rate has little effect on the reaction rate and brings about a higher pressure drop. The pyrolysis under different well patterns is also compared and the results indicate that four-well heating and six-well heating modes can improve the uniformity and the reaction conversion rate with the same total inlet mass flow rate during the same time. For the six-well heating mode, the pyrolysis reaction can be completed in about 100 days, which is 60 days shorter than the single-well heating mode.

Polyethylene glycol with dual three-dimensional porous carbon nanotube/diamond: a high thermal conductivity of composite PCM

Polyethylene glycol (PEG) is widely used as a phase change material (PCM) in thermal energy storage systems due to its high latent heat and chemical stability. However, practical application has been hindered by its low thermal conductivity and leakage issues. Therefore, developing shape-stable high thermal conductivity PCM is of great importance. In this study, new shape-stable composite PCM with high thermal conductivity and leak-prevention capabilities were designed. The porous carbon skeleton of diamond foam (DF) and dual-3D carbon nanotube-diamond foam (CDF) were prepared using the microwave plasma chemical vapor deposition method. The composite materials (DF/PEG and CDF/PEG) were produced by vacuum impregnation with PEG and skeletons. The results showed that CDF/PEG had the highest thermal conductivity, measuring 2.30 W·m−1·K−1, which is 707% higher than that of pure PEG. The employing of 3D networks of CNTs, which can improve the phonon mean free path in DF/PEG (1.79 W·m−1·K−1) while reducing phonon dispersion.The phonon vibration of dual-3D CDF plays an important role in heat transfer. PEG was physically absorbed and well-distributed in CDF, alleviating leakage of liquid PEG. The weight loss of CDF/PEG was only 25% at 70 °C for 120 s. Using CDF is an attractive and efficient strategy to increase the heat transfer of PEG and improve heat storage efficiency, alleviate the problem of poor shape-stability.

BEHAVIOR OF NON-NEWTONIAN FLUID IN A LID-DRIVEN CHAMBER WITH HEATED BODIES SUBJECTED TO A UNIFORM MAGNETIC FIELD

Recent studies have attempted to find methods and techniques that enable the development of heat transfer within thermal transformers. For this purpose, this paper includes a numerical simulation of a non-Newtonian fluid inside a chamber with four hot bodies. The chamber is exposed to a magnetic field of constant and uniform intensity. Also, the upper wall of the chamber has a horizontal movement. The study is based on the quality of heat transfer between the heated parts of the chamber and the complex fluid under the influence of a set of criteria, namely Reynolds number (= 1 to 40); Hartmann number (= 0 to 100); power-law index (= 0.6 to 1.4) and Richardson number (0 to 100). The interpretation of the results is done by displaying the path-lines and isotherms distribution. Also, the amount of thermal transfer of the hot bodies is given in terms of the Nusselt number. The results showed that the positioning of the hot bodies plays an important role in heat transfer. Moreover, increasing the value of the power-law index makes the fluid stickier, which reflects its negative effect on heat transfer.

Numerical investigation on heat transfer and vapour layer geometry of a droplet impacting on a spherical particle in Leidenfrost regime

Understanding the heat transfer mechanism and dynamic behaviors of droplet-particle collision in Leidenfrost regime is essential for the pyrohydrolysis process of pickling liquor, which is sprayed into a fluidized bed and then decomposes to oxides and acid vapour. In this study, the droplet deformation, heat transfer characteristics and vapour layer geometry of Leidenfrost droplet impacting spherical particle are investigated by constructing a microscale phase change model in VOF framework. The applicability of the computational model in capturing droplet dynamics and thermodynamics is verified by comparing it with experimental outcomes. The results indicate that the vapour layer geometry is relatively uniform in spreading but shows instability in recoiling. The vapour layer thickness increases rapidly in both spreading and early recoiling, while decreases in the late recoiling phase. Furthermore, the heat transfer and droplet evaporation depend on the vapour layer thickness strongly. Specifically, the thinner the vapour layer, the lower the heat transfer resistance. The increasing surface temperature, We and Re can effectively enhance heat transfer and raise droplet temperature. In addition, the maximum spreading factor and liquid–solid contact time play an important role in heat transfer and they both show a clear power-law dependency on We and Re for viscous droplets. The effect of droplet size on contact time is consistent with first-order vibration theory but the contact time is independent of surface temperature and impact velocity.

The Investigation of Indoor Air Temperature Effects for Different Types of Window Dimensions Using CFD Simulation

Windows plays an important role in heat transfer and natural ventilation in buildings. In the field of building design, the challenge is to provide a thermally comfortable indoor environment that requires the least energy consumption to maintain. One of the energy reduction alternatives that can be incorporated into modern building design is the installation of efficient windows. The windows should allow thermally acceptable indoor air quality and have a pleasant design while being energy efficient. Good thermal conditions in school buildings, especially learning spaces, promote the educational process. Unfulfilling comfort levels can reduce the lecturers’ and students' physical and intellectual performance. One of the variables that affect the thermal comfort condition is indoor air temperature. The purpose of this study is to investigate the effect of window size on indoor air temperature by using Computational Fluid Dynamics (CFD) simulation. The selected classroom at Universiti Teknologi MARA (UiTM) Permatang Pauh, Pulau Pinang will be used as a case study. The field measurement result can be used to validate the simulation result. The simulation result will be compared to standard regulations set by the American Society of Heating, Refrigeration, and Air Conditioning Engineers (ASHRAE) Standard 55-2017. Three different sizes of windows have been chosen in this study which are double, triple, and quadruple-glazed windows. The validation result shows the percentage difference in indoor air temperature between the experimental and simulation is 1.04 %. The result found double and triple-glazed windows show the value of temperature under the range set by ASHRAE Standard 55-2017. The selection of triple-glazed windows is more suitable because the brightness of the classroom meets the criteria for the absence of direct solar radiation. In conclusion, the size of the window shows the performance in terms of heat transfer and indoor air temperature of the classroom.

A comparative study of fluid flow and heat transfer in the tube with multi-V-winglets vortex generators

In this paper, a new multi-V-winglets vortex generator was proposed and its influence on fluid flow and thermal transfer was studied. It was found that the thermal transfer was enhanced with the increasing number of winglets, while the friction resistance was growing. Therefore, punching holes on the surface of vortex generators were used to enhance overall performance by reducing friction resistance. The simulation results showed that jet flow appeared when fluid passed through hole, which impacted the recirculation zone and removed the stagnant fluid. Thus, local heat transfer was enhanced and friction loss was decreased. Meanwhile, the pitch of multi-V-winglets vortex generator had an important role in heat transfer and friction loss, which could increased the Nusseltnumber of the smooth tube by 130.57–156.42%. Furthermore, the results were proved using the principle of entropy generation. The maximum performanceevaluationfactor was 2.83 when the number of winglets was 8, the holes were on the outer surface of winglets and the pitch was 25mm.

The effect of coupled combustion reaction on conjugate heat transfer in boundary layer of a segregated oxidizer/fuel solid motor

Combustion reaction plays an important role in heat transfer of boundary layer of solid rocket motor. In the present study, a transient numerical model is developed to investigate the influence of coupled combustion reaction on heat transfer in boundary layer of the segregated oxidizer/fuel solid motor (SOFSM). The results show that conjugate heat transfer characteristics of non-reacting flow are distinct from those of the reacting flow with coupled combustion. The reacting flow with coupled combustion renders higher and more homogeneous temperature and velocity compared with the non-reacting flow. Due to the effect of combustion heat release on thermophysical properties of the gas phase species, the heat transfer coefficient and effective thermal conductivity of the reacting flow are much higher than those of the non-reacting flow. When the combustion time increases, the heat transfer coefficients decrease in the reacting and non-reacting flows due to reduced flow velocity and lower O2 concentration. The Nusselt number (Nu) of the reacting flow is also higher than that of the non-reacting flow. Significant decrease of Nu is found in the non-reacting flow with combustion time. However, the Nu varies very slightly along the flow direction in the reacting flow. As pressure increases from 1.5 MPa to 5.0 MPa, the high-temperature area becomes narrower and the flow velocity decreases in the combustion channel. The diffusion and mixing of the oxidizer and fuel gas along the radial direction are thus weakened in the coupled combustion flow. The total heat transfer coefficient decreases from 295.88 W m−2 K−1 to 46.52 W m−2 K−1 and Nu decreases from 36.27 to 16.56 with increase of pressure. The distribution of total heat transfer coefficient is found to be highly synchronous to the distribution of regression rate of coupled combustion process. The distribution of heat transfer coefficient directly affects parallel regression and coupled combustion of the TAGN propellant. The fitting relationship of Nu, i.e., Nu = 0.058Re0.6Pr0.424 is obtained for the coupled combustion flow in the motor, which provides better insight into the heat transfer in the SOFSM system.

Study on the heat transfer characteristics of a plate-fin-typed precooler considering cooling fluid phase change

A quasi-two-dimensional calculation method has been proposed to design a plate-fin-typed precooler with high efficiency and low resistance. This method considers the variation of fluid properties during the heat transfer process, particularly for cryogenics with phase change. The precooler heat transfer characteristics are comprehensively investigated based on the proposed method and calculation models. The calculation results demonstrate that the proposed method effectively explores the fluid properties distribution, such as the temperature distribution and steam quality distribution inside the precooler. Besides, the hot fluid temperature drop exceeds 1200 K, while the pressure drop remains below 15% of the inlet pressure above all cases. Moreover, the heat transfer performance between cooling fluid and hot fluid is compared. It is worth noting that the cooling fluid heat transfer coefficient is10∼102 magnitude higher than that of the hot fluid, especially when the cooling fluid steam quality is below 0.60. Therefore, it is critical to improve the heat transfer performance of the hot fluid for the precooler. Additionally, the first heat exchanger inside the precooler plays a more important role in heat transfer than heat exchangers at other locations because the temperature drop and pressure drop inside the precooler account for the largest proportion, with a value up to 50%.

High Spatial Resolution Thermal Mapping of Volatile Switching in NbOx-Based Memristor Using In Situ Scanning Thermal Microscopy.

Temperature mapping by in situ thermoreflectance thermal imaging (TRTI) or midwave infrared spectroscopy has played an important role in understanding the origins of threshold switching and the effect of insulator-metal transitions in oxide-based memrsitive devices. In this study, we use scanning thermal microscopy (SThM) as an alternative thermal mapping technique that offers high spatial resolution imaging (∼100 nm) and is independent of material. Specifically, SThM is used to map the temperature distribution in NbOx-based cross-bar and nanovia devices with Pt top electrodes. The measurements on cross-bar devices reproduce the current redistribution and confinement processes previously observed by TRTI but without the need to coat the electrodes with a material of high thermo-reflectance coefficient (e.g., Au), while those on the nanovia devices highlight the spatial resolution of the technique. The measured temperature distributions are compared with those obtained from physics-based finite-element simulations and suggest that thermal boundary resistance plays an important role in heat transfer between the active device volume and the top electrode.

Comparative experimental study on dynamic characteristics of bubble microlayers in small channel flow boiling and pool boiling

The microlayer present at the bottom of the bubble plays a very important role in heat transfer during nucleation boiling. In this paper, the dynamic characteristics of microlayer at the bottom of boiling bubble in a small channel were studied by laser interferometry method and high speed camera, and the results were compared with pool boiling experiments. The results show that the bubbles have an obvious tendency to slip in the flow boiling. The microlayer interference fringe is deformed and no longer a complete Newtonian ring. By analyzing the thickness distribution of microlayer in different directions in flow boiling and the change of micro-contact angle in the flow direction, it was found that the micro-contact angle in the flow direction became larger with the growth of bubbles.

Lattice Boltzmann simulation of fluid flow and heat transfer in a micro channel with heat sources located on the walls

This study investigated a numerical study of fluid flow and heat transfer in micro channel with four heat sources that located on upper and lower walls. Four heat sources are located in the upper and lower walls symmetrically respect to centerline. Lattice Boltzmann method is used to solve related equations of flow and temperature of fluids, this method is included two steps such as collision and streaming steps. Reynolds number is varied from 0.1 to 10 and Knudsen number is changed from 0 to 0.1 for air. The slip velocity and temperature jump boundary condition are used for micro channel simulation with Knudsen numbers related to slip velocity flow. Bounce-back boundary conditions were applied on all solid boundaries, which means that incoming boundary populations are equal to out-going populations after the collision. A comparison with other study is done and a good result is gained. The results show that the Knudsen number has important role in heat transfer and the highest mean temperature at outlet occurs at highest Knudsen number and heat transfer convection is more significant for first heat source comparing second heat source.

Mixed convection heat transfer of molten salt outside coiled tube

Mixed convection heat transfer of molten salt outside coiled tube in cylinder tank was experimentally reported with Reynolds number 418–2175 and Prandlt number 5.7–8.8. Nusselt number of salt outside coiled tube was significantly higher than that inside tube, and its tendency had good agreement with available heat transfer correlations for fluid crossing tube. The heat transfer between salt and coiled tube was enhanced by forced convection, and its Nusselt number rose as Reynolds number rising. Based on available heat transfer correlations and experimental data, experimental correlation for forced convection was proposed by considering Prandtl number ratio, and it fitted with experimental data with maximum deviation of 10.1%. Natural convection played important role in heat transfer outside coiled tube for large temperature difference, and its effect increased as Reynolds number dropping. In addition, experimental correlation for mixed heat convection with combination of natural convection and forced convection was proposed, and it fitted with experimental data better than other correlations, and maximum deviation was reduced to 8.5%.

Evaluation of developed alkali-activated bricks for energy-efficient building construction

Gradually, the temperature of the earth is increasing due to global warming, which has increased the operational energy demand in buildings. This energy usage can substantially be reduced using energy-efficient measures such as developing a thermally resistant building envelope. The building envelope mainly consists of walls, roof and floor, where walls play an important role in heat transfer that is a major contributor to operational energy demand. In this study, a walling material (alkali-activated bricks) was developed, where locally available industrial wastes (co-fired blended ash and stone dust (CBA-SD)) were activated by a sodium-based alkali activator and cured at an ambient temperature of around 37°C. The alkali-activated bricks were evaluated for mechanical-, physical-, thermal- and durability-related properties that are in accordance with Indian standards. The results showed that the developed alkali-activated bricks are lighter in weight and have lower thermal conductivity (0.35–0.4 W/(m K)) as compared to conventional fly-ash (FA) bricks (1.05 W/(m K)). A case of a residential building with varying building envelope and number of floors was simulated for analysing building performance using a computational approach. As compared to FA bricks, the CBA-SD brick model resulted in peak cooling load reduction by 49–51, 46–48 and 44–46% for single-, two- and three-storeyed buildings, respectively.

Experimental investigation of heat transfer in pin-fins heat sinks for cooling applications

This study investigates the thermal performance of square pin-fin heat sinks for different operating conditions including different heat flux and different Reynolds numbers. Two heat sinks with different sizes of copper square pin-fins cross section (2-mm fins and 500-μm fins) in-line arranged are experimentally investigated to determine their thermal performance quantified in terms of thermal resistance. The investigation is carried out for a Reynolds numbers between 80 and 470, and heating rate between 21.9–46.7 W. The results indicate that the thermal resistance is lower for micrometer sized fins for smaller Reynolds number as compared to millimeter sized. However, for the larger Reynold numbers, thermal resistance of millimeter sized fins was observed to be lower. The fluid velocity plays a very important role in heat transfer, and it has more pronounced effect on the thermal resistance compared to the fins size.

A rigorous model for prediction of viscosity of oil-based hybrid nanofluids

Oil-based hybrid nanofluids play an important role in heat transfer in cooling systems and lubrication. Therefore, various experimental investigations are conducted to estimate their viscosity. However, such measurements can be carried out on limited types of oil-based hybrid nanofluids and often are time consuming and expensive. The main objective of this paper is to develop a rigorous data-driven method based on an advanced genetic programming (GP) called multigene genetic programming (MGGP) to predict the viscosity of Newtonian oil-based hybrid nanofluids which has not previously been used in this area. A comparative analysis was performed using the gene expression programming (GEP), multi-variate linear regression (MLR) methods and various correlations. 679 experimental data points with different nanoparticles and oil-based fluids were collected from literature to develop the Artificial Intelligent (AI) models. The new approach showed superior performance in estimating of the relative viscosity of oil-based hybrid nanofluids in comparison with all correlations methods. Furthermore, the MGGP results for the test dataset (R=0.991, RMSE=0.05, PI=0.643) were more accurate than those obtained from the GEP (R=0.975, RMSE=0.083, PI=0.696) and MLR (R=0.912, RMSE =0.153, PI=1), respectively. The sensitivity analysis was also performed demonstrating that the volume fraction (PIs=0.849, DV1=10.079%), temperature (PIs=0.463, DV2=9.966%) and nanoparticles size (PIs=0.420, DV3=6.092%) are the most significant factors in assessing relative viscosity, respectively.

Enhancement of subcooled boiling in confined space using ultrasonic waves

Effects of ultrasonic waves on heat transfer and bubble behavior spanning from nucleate boiling to film boiling or microbubble emission boiling (MEB) in a confined space were investigated. Experimental results show that critical heat flux in the confined space is increased by four times in the ultrasonic field. In transition boiling regime, MEB occurs at subcooling over 20 K in the unconfined space, however it cannot be observed in the confined space even for a subcooling of 60 K. Visualizations show that a large coalesced bubble traps in the confined space, which inhibits the supply of liquid onto the heating surface. Applying the ultrasound will reintroduce MEB at subcooling over 25 K. A comparison of MEB in the confined space with and without the ultrasound reveals that the strong bulk convection introduced by the violent oscillation and collapse of vapor film plays an important role in heat transfer of MEB.

A Detailed Finite Element Model of Internal Short Circuit and Venting During Thermal Runaway in a 32650 Lithium-Ion Battery

The frequent accidents of power lithium-ion battery have become the major reason to hinder the development of electric vehicles. In this paper, the thermal runaway process for a 32650 battery is analyzed based on 300°C oven heating experiment in adiabatic rate calorimeter, the rise of temperature, the drop of voltage and the leakage of electrolyte are observed before exploding, which could be used as predictor variables for thermal runaway warning. A large number of smoke releases and diffuses after explosion, which could be utilized as a criterion for determining the explosion. And a lumped chemical reaction kinetics model coupled with three-dimensional heat transfer model is constructed for further discussion. The thermal runaway process of the battery could be accurately calculated by the coupled model. Thermal radiation plays a more important role in heat transfer than heat convection in the process of thermal runaway. The explosion happens when the temperature achieves around 230°C, and the active material mainly starts to decompose at this moment.

Role of gravity in condensation flow of R1234ze(E) inside horizontal mini/macro-channels

The condensation patterns of R1234ze(E) inside horizontal mini/macro-channels were numerically investigated under normal-gravity and zero-gravity conditions. The gravity effects on condensation heat transfer coefficients, liquid film thickness, film distribution, cross-sectional stream-traces, and liquid-phase velocity were analyzed detailedly. The influence of surface tension on condensation flow was also discussed. The gravity effect on condensation heat transfer coefficients was negligible in mini-channels with D = 1 mm, while was important for D = 2 mm and D = 4.57 mm. The gravity effect can either enhance or weaken the condensation heat transfer coefficient, which was dependent on the tube diameter and vapor quality. The enhancement on heat transfer caused by the gravity was more pronounced at lower vapor quality and mass fluxes with a larger diameter tube. The gravity affected the condensation heat performance through changing the vapor-liquid distribution, rather than the film thickness. The gravity has a great influence on the condensation flow field in both circumferential and axial direction. The surface tension played an important role in heat transfer under zero-gravity condition.

Thermal Characterization of Convective Heat Transfer in Microwires Based on Modified Steady State “Hot Wire” Method

The convection plays a very important role in heat transfer when MEMS work under air environment. However, traditional measurements of convection heat transfer coefficient require the knowledge of thermal conductivity, which makes measurements complex. In this work, a modified steady state hot wire (MSSHW) method is proposed, which can measure the heat transfer coefficient of microwires' convection without the knowledge of thermal conductivity. To verify MSSHW method, the convection heat transfer coefficient of platinum microwires was measured in the atmosphere, whose value is in good agreement with values by both traditional measurement methods and empirical equations. Then, the convection heat transfer coefficient of microwires with different materials and diameters were measured by MSSHW. It is found that the convection heat transfer coefficient of microwire is not sensitive on materials, while it increases from 86 W/(m$^2$K) to 427 W/(m$^2$K) with the diameter of microwires decreasing from 120 ${\mu}$m to 20 ${\mu}$m. Without knowing thermal conductivity of microwires, the MSSHW method provides a more convenient way to measure the convective effect.