As research continues into the generation IV advanced nuclear reactors, exploration of liquid sodium as a coolant, or Sodium Fast Reactors (SFRs), coupled to supercritical CO2 (sCO2) Brayton cycles are currently underway. Liquid sodium offers unique and beneficial fluid properties that can achieve higher efficiencies and longer equipment lifespans compared to conventional water cooled reactors. Coupling sodium with sCO2 matches well with sodium’s temperature profile and is less reactive with sodium when compared to water used in standard Rankine cycles. To achieve commercial viability, methods for developing diffusion-bonded Hybrid Compact Heat Exchangers (H-CHX) to couple SFRs with sCO2 Brayton cycles are being developed. This paper includes thermal-hydraulic analysis of these fluids to quantify thermal and pressure stresses within the H-CHX for use in determining a structurally sound design. Two models for predicting the temperature profiles within a practical H-CHX channel design are presented. The first is a 1-D heat transfer model employing heat transfer correlations to provide both bulk fluid and wall temperatures. The second is a 3-D computational fluid dynamics model (CFD) providing a three-dimensional temperature profile, but at a significantly increased simulation time. By comparing the results of the two models for specific design conditions, significant temperature deviation is shown between the models at a short channel length of 10 cm. However, for longer channel lengths, although the 1-D model neglected the strong axial conduction on the sodium side, it generally shows good agreement with the CFD model. Thus, for any practical H-CHX designs, the findings reveal both simulation methods can be used to extrapolate the temperature gradient along the channel length for use in designing a H-CHX, as well as predicting the overall size and mass of the heat exchanger for component costing.

Lithium-ion (Li-ion) batteries have been increasingly used in electric vehicles (EVs) in the past ten years due to their high energy and power density. However, the poor performance at low temperatures is the main challenge to the deployment of EVs equipped with Li-ion batteries because it has great influence on the driving range. In this study, conductive heating method is proposed to improve the lithium ion battery performance at low temperatures (−20°C ∼ −5°C). Both experimental testing and numerical simulation are employed to compare different heating protocols to minimize the temperature gradient, external heating power and heating time. Results show that simultaneous heating with small external power and high discharge voltage can obtain optimal performance as compared to preheating protocol. In addition, internal heating combined with external heating could heat up the battery from −20°C to 0°C in only 1 minute which is an effective strategy.

Nearly 75% of energy produced by fuel is eventually rejected to the environment and ultimately goes unused in terms of waste heat in motor vehicles. A promising method of reclaiming energy waste is to use thermoelectric (TE) energy harvesters which are multi-material solid-state devices that convert a thermal gradient directly into electric potential. In current automotive applications, waste heat recovery systems using TE are only limited to integration on exhaust pipes to convert hot exhaust gases into electricity. In this study, we explored the use of TE materials in the shape of a car oil pan to utilize the temperature difference of hot engine oil and cool outside air and convert this temperature gradient into electricity. In this study, we performed finite element simulations to optimize the geometry and the quantity of thermoelectric modules. This optimization was performed to achieve maximum thermoelectric power under the constraints of manufacturability. Using these optimum design parameters, we determined that 2.3 kW output power can be recovered from the flat plate oil pan and 2.6 kW from the oil pan with a single step due to the enhanced surface area. These power amounts were found to be higher than those previously obtained from thermoelectric systems integrated to exhaust pipes.

Thermoelectric (TE) energy harvesters are multi-material solid-state devices that convert heat (i.e. a thermal gradient) directly into electric potential. Currently, the biggest challenge limiting the applications of thermoelectric devices is the low conversion efficiency (< 10%). To achieve higher thermoelectric efficiency, electrical conductivity and Seebeck coefficient of thermoelectric materials must be maximized allowing the flow of charge carriers and thermal conductivity must be minimized keeping high temperature gradient between hot and cold sides. These properties are strongly coupled to each other. In other words, improving one property deteriorates the other. In nanoscale however, manipulation of matter at the atomic level can decouple these properties. Nanoengineering is therefore considered to be the only remedy for the low conversion efficiency of thermoelectric materials. Current nanomanipulation techniques focus only on reducing thermal conductivity by scattering heat carrying phonons with nanoscale artifacts. We have observed that doping thermoelectric material with carbon quantum dots (size < 5 nm) tremendously increased electrical conductivity and thermoelectric power. In the control experiments using carbon powder (same chemical arrangement but larger scale, < 100 nm), we did not observe any increase in thermal power density evidencing the nanomanipulation of material properties using carbon quantum dots. Doping thermoelectric materials with carbon quantum dots has high potential due to the quantum enhancement effects on electrical properties of and needs to be further investigated for the design of novel nanocomposite materials with superior thermoelectrical properties.

This experiment measures the temperature and the velocity field synchronously in the boundary layer in a rotating smooth, wall-heated channel using hot-wire. The Reynolds number based on the bulk mean velocity and hydraulic diameter is 19000 and the rotation numbers are 0, 0.07, 0.14, 0.21, 0.28 and 0.35. Four streamwise stations (X/D = 4.06, 5.31, 6.56, 7.81) were investigated. To calibrate the parallel-array hot-wire probe, a heating section is added to the original wind tunnel that could only calibrate the hot-wire at room temperature. Different gas temperatures at the outlet could be obtained by changing the heating power of the heating section. The velocity profiles and the temperature profiles are obtained. It can be seen that the viscous sublayer also exists when the wall is heated, thus the viscous sublayer profile method is also valid when the wall is heated. It is found that the velocity profile near the leading side is more sensitive to the change of rotation number and X/D than the velocity profile near the trailing edge. The critical rotation number phenomenon of velocity profile has also been found in present work. By comparing with the previous work without the wall heated, the influence of both kinds of buoyancy under this condition is discussed. Some explanations are given for the experimental results.

Li-ion based energy storage devices have highly temperature dependent characteristics such as performance, life-cycle, efficiency and safety. Large temperature gradient within a cell results in thermal stresses and nonuniform current density leading to accelerated degradation. This adversely affects the life cycle of the cell due to capacity and power fade. There are similar issues due to large temperature variation within a battery pack. Operation of Li-ion cell outside the desirable temperature range also leads to lower efficiency, degradation and safety related issues. Different thermal management approaches have been proposed and demonstrated in past. The present work focuses specifically on minichannel based liquid cooling for conducting a parametric study. Minichannels have been found effective in various thermal management applications due to their simple construction and high convective heat transfer. In past, minichannels have been proposed and used in battery thermal management. However, designing of such systems has been somewhat arbitrary without considering various factors and trade-offs involved. There is a lack of rigorous studies for determining various parameters related to thermal management system that would result in adequate thermal management in a cost-effective manner. In the present work, a comprehensive parametric study has been carried out on the minichannel based liquid cooling for thermal management of Li-ion battery pack. A simplified computationally efficient numerical simulation-based approach has been used to conduct parametric study for optimizing the design and operating parameters of the thermal management system.

The complicated steel wheel and rail interaction on curve causes side wear on rail head. Thus, the cost of maintenance for the track on curve is significantly higher than that for track on a tangent. The objective of this research is to develop 3D printing technology for repairing the side wear. In this paper, the study examines induced residual thermal stresses on a rail during the cooling down process after 3D printing procedure using the coupled finite volume and finite element method for thermal and mechanical analysis respectively. The interface of the railhead and additive materials should conserve high stresses to prevent any crack initiation. Otherwise, the additive layer would likely shear off the rail due to crack propagation at the rail/additive interface. In the numerical analysis, a cut of 75-lb ASCE (American Society of Civil Engineers) worn rail is used as a specimen, for which a three-dimensional model is developed. The applied residual stresses, as a result of temperature gradient and thermal expansion coefficient mismatch between additive and rail materials, are investigated. At the beginning, the worn rail is at room temperature while the additive part is at a high initial temperature. Then, additive materials start to flow thermal energy into the worn rail and the ambient. The thermal distribution results from thermal analysis are then employed as thermal loads in the mechanical analysis to determine the von-Mises stress distribution as the decisive component. Then, the effect of preheating on residual stress distribution is studied. In this way, the thermo-mechanical analysis is repeated with an increase in railhead’s initial temperature. In thermal analysis, the temperature contours at different time steps for both the non-preheated and preheated cases indicate that preheating presents remarkably lower temperature gradient between rail and additive part and also represents a more gradual cooling down process to allow enough time for thermal expansion mismatch alignment. In mechanical analysis, the transversal von-Mises stress distribution at rail/additive interface is developed for all cases for comparison purposes. It is shown that preheating is a key factor to significantly reduce residual stresses by about 40% at all points along transversal direction of interface.

The increasing demand for system miniaturization and high power density energy produces excessive thermal loads on electronic devices with significant mechanical strain. Carbon Nanotubes (CNTs) based devices are found to have excellent thermal transport properties that makes them attractive for thermal management of these miniaturized nano-electronic devices under extreme environments. These conductive nanostructure (carbon nanotubes, graphene, etc.) are often embedded in polymers or other high-strain alloys (the matrix phase), and are used as bridging materials for conductivity (electrical and thermal) with strain resiliency. The effect of strain on the thermal transport properties of these nanostructures have often been overlooked and will be the focus of this work. The thermal conductivity of the nanostructure is obtained in LAMMPS using the Heat-Bath method, which is a reverse non-equilibrium molecular dynamics (RNEMD) simulation strategy. In RNEMD, constant amount of heat is added to and removed from hot and cold regions and the resultant temperature gradient is measured. The effect of strain on the thermal conductivity of the single and multiwalled nanostructures of various configurations will be discussed with specific emphasis on the phonon density of states of nanotubes at different strain states.

Laser welding is used to join metals of small thickness. The heat affected zone in this process is very small and the temperature gradients encountered are very large (of the order of 10 4 K/mm ). We have used numerical methods to analyze the effect of various parameters. For the present study, an in-house C code was developed to model the laser welding process. A Finite Volume based discretization was done and a Semi-Explicit method was used to solve the governing equations. Conduction mode heat transfer was considered by assuming no vaporization of the molten metal. The effect of various parameters like beam intensity and beam radius were studied on the weld dimensions. The beam intensity was varied from 5.2KW to 15KW while the beam radius was varied from 0.14mm to 0.35mm. The effect of welding angle was also studied on the weld dimensions. The welding angles used for the study were 0°, 26.5°, 45° and 56°. It was found that the weld width and depth increased with an increase in beam intensity and a decrease in beam radius, while the weld depth decreased with an increase in welding angle. The change in heat transfer and fluid flow was also studied by varying these parameters.

The purpose of this study is to investigate the dynamic behavior of a Solid Oxide Steam Electrolyzer (SOSE) system without an external heat source which uses transient photovoltaic (PV) generated power as an input to produce compressed (to 3MPa) renewable hydrogen to be injected directly into the natural gas network. A cathode-supported crossflow planar Solid Oxide Electrolysis (SOE) cell is modeled in a quasi-3D thermo-electrochemical model that spatially and temporally simulates the performance of a unit cell operating dynamically. The stack is comprised of 2500 unit cells that are assumed to be assembled into identically operating stacks, creating a 300kW electrolyzer stack module. A 15-minute resolution dataset for operation of PV generation was obtained from a database that archives PV power dynamics of systems on the University of California, Irvine campus. The dataset (comprised of data for approximately 4.1 MW of peak solar power) was scaled to a maximum of 450kW of PV generation. For the designed 300kW SOSE stack (thermoneutral voltage achieved at design steady state conditions), powered by the dynamic 0–450kW output of PV systems, thermal management and balancing of all heat supply and cooling demands is required based upon the operating voltage to enable efficient operation and prevent degradation of the SOSE stacks. The PV generation dataset was analyzed to obtain a day in which the PV generated power has its highest dynamic behavior (a cloudy day) and another day in which the PV generated power and energy is maximum (a sunny day). Dynamic system simulation results show that the SOSE system is capable of following the dynamic PV generated power for both of these days while the SOSE stack temperature gradient is always maintained below a maximum set point along the stack for both days. The system efficiency based upon lower heating value of the generated hydrogen is between 0–75% and 0–78% with daily hydrogen production of 94kg and 55kg for sunny and cloudy days, respectively.

Effective heat transfer techniques benefit the development of nuclear and fossil fuel powered steam generators, high power electronic devices, and industrial refrigeration systems. Boiling dissipates large heat fluxes while keeping a low and a constant surface temperature. However, studies of the fluid behavior surrounding the bubble and the heat transfer near the contact-line are scare due to difficulties of flow visualization, chaotic conditions, and small length scales. The preset study shows the simulation of bubble growth over a heated surface from conception to departure. The computation of mass transfer with interfacial temperature gradients leads to proper bubble growth rates. Models to include the interface sharpness uncover the dynamic and thermal interaction between the interface and the fluid. Results indicate that the nucleation of a bubble (in water at 1 atm with 6.2 K wall superheat) has an influence region of 2 D b (where D b is the departure bubble diameter). In addition, results reveal a thin thermal film near the interface that increases the heat transfer at the contact-line region. Numerical bubble growth rates compare well with experimental data on single bubble nucleation.

Insulator-based dielectrophoresis (iDEP) is known as a powerful technique for separation and manipulation of bioparticles, using arrays of insulating posts and external electrical field. In this research, we utilized numerical simulation to study, in detail, the Joule heating which is one the most important phenomena in iDEP technique specially related to bioparticles separation and manipulation in physiological samples. Although Joule heating has been observed in both electrode-based and insulator dielectrophoresis, its effect is more significant in iDEP since higher electric potentials are required in this technology. As a result of the external electrical field, the temperature gradients would create conductivity, permittivity, viscosity and density local gradients in the solution, and consequently cause bulk fluid forces and fluid motion, known as electrothermal flow (ET). These flow circulations can cause unpredicted behavior of the device and even cause problems due to clogging. Moreover, the temperature rise due to the Joule heating could threaten the cell viability. In this study, we are going to develop a robust numerical model for predicting the flow behavior in the existence of external electric field and determining the temperature and velocity profile which can determine the cell viability and clogging problem in iDEP microdevices. The developed numerical tool was used based on the properties of circulating tumor cells (CTCs) and White blood cells (WBCs) and their separation.

All sealing technologies present inconveniences to be solved. One of the most significant adverse effects in the sealing production lines is the amount of material waste that is genetated due to the difficulties of achieving the correct thermal phase change in the polymer film during the sealing process. Sealing methods such as Induction, ultrasonic or heated jaw have similar operating characteristics, minimum gradients in pressure and temperature that are nedeed to create an efficient polymer transition, to produce the correct molecular arrangements and to avoid zones with little or null interaction between melted surfaces. This review shows the factors that affect the performance of jaw-multilayer polymer films that produce bad seal integrity; through understanding the design parameters to generate a small pressure and temperature gradients also characterizing and testing this phenomenon, the result of the review will help to identify theoretical basis and opportunities to design an innovate heated jaw sealing system with a uniform pressure and temperature along the sealing area.

In this work, the thermal simulation of dissimilar fusion welding system is demonstrated by considering the phase lag effects in ultra-short pulse laser source. When the pulse duration is comparable with the electron relaxation time, the hyperbolic effect cannot be neglected in heat transfer analysis due to femtosecond laser. The non-Fourier effect is considered for heat transfer analysis assuming finite delay in development of temperature within the body. This delay is represented in terms of relaxation times connected to heat flux and temperature gradient. In the present work, the simulation has been proposed by developing 3D finite element based heat transfer model using dual phase lag effect. Since the experimental basis of transient temperature distribution in ultra-short pulse laser is extremely difficult or nearly impossible, the model results have been validated with literature reported results. The model has been used further for the simulation of temperature distribution in femtosecond fiber laser welding of dissimilar aluminum alloy and stainless steel. The results in terms of computed isotherm are compared with experimentally evaluated weld pool geometry for dissimilar materials from independent literature. The influence of other characteristic parameters like pulse frequency, pulse width and relaxation times are assessed for this simulation based study which will effectively reduce the costly experimental effort for differential influence of process parameters. A clear guideline of geometric shape and size of weld pool geometry and peak temperature of the welding system with reference to predictable laser parameters are the effective output of this simulation based study. It was observed that the peak temperature reached in a very short interval of time, in the order of nano-seconds. Such high heating or cooling rate impacts on the microstructural changes of the welded joint. In order to reach certain temperature, multiple pulses are required in the material processing of either very thin film or microwelding to keep the thermal shock distortion as low as possible.

Paddy drying is one of the main operations on the rice milling industry and it has a wide impact in terms of kernel quality, infestation reduction and costs of operation. That is why process control is one of the main fields to ensure a successful operation. This paper presents the development of an industrial scale drying rate prediction model. The model includes on his structure mass balance equations, drying rate and bulk density equation and an experimental expression that estimates the rate of change in moisture as a function of the dry basis kernel water fraction, Sherwood number, mass transfer Biot number and a proposed non-dimensional temperature -Nt-. The importance of including -Nt- parameter is based on the thermal gradients between the drying air, internal kernel water and the installation environmental conditions. These factors create a distinctive characteristic from other drying rate prediction models because they associate the process with environmental conditions of the location where drying operation takes place.

This paper presents the results of an experimental study on buoyancy induced convection in open cell aluminium foams attached to a one sided heated plate. The foam samples had porosity of around 90% and pore densities of 2, 4, 8 and 16 pores per cm. (PPC). The experiments were conducted under different orientations ranging from horizontal (heated from below) to vertical (heated from side). The heat transfer rates from the heater plate was measured and found to be a strong function of pore density and orientation as well as temperature gradient. In all the cases, the heat transfer rate was found to be superior to that of a vertical flat plate. For a given porosity and height the samples with larger pore sizes were found to be more effective. Finally the effective heat transfer coefficient was plotted in the form of Nusselt number as a function of Rayleigh number to capture its dependence on the foam geometry and boundary condition.

In this work numerical experiment was performed for studying the heat transfer and thermodynamic performance of the melting process in a bottom heated square cavity. The bottom wall is maintained at a temperature higher that the melting temperature of the PCM, while all other walls are perfectly insulated. The transient numerical simulations were performed for melting Gallium, (a low Prandtl number PCM with high latent heat to density ratio) at moderate Rayleigh number ( Ra ≊ 10 5 ). The transient numerical model consist of solving coupled continuity, momentum and energy equation in the unstructured formulation using the PISO algorithm. In this work, the fixed grid, source-based enthalpy-porosity approach has been adopted. The heat transfer performance of the melting process was analyzed by studying the evolution of global fluid fraction, Nusselt number at the hot wall, volume averaged normalised flow kinetic energy with time. The thermodynamic performance is analyzed by calculating local entropy generation rates considering both irreversibility due to finite temperature gradient and viscous dissipation. The values of second law efficiency clearly shows that the current thermal design of the phase-change heat accumulators are very close to the ideal design.

Recent research has identified Adini’s rectangular element as an efficient higher order element for solving second order elliptic partial differential equations such as Poisson’s equation, which governs the steady state heat conduction problem. This type of element includes both the primary field variable and its spatial derivatives as nodal degrees of freedom. Compared to the conventional cubic elements of the Serendipity and Lagrange families, Adini’s element includes the minimum number of nodes per element and has the advantage that the nodal values of the spatial derivatives of the temperature field are directly retrieved from the FEM solution. As a result, the differentiation and averaging procedures that are typically used to obtain the nodal values of the temperature gradients are avoided. In this paper a generalized version of Adini’s element for solving two-dimensional steady state heat transfer problems in non-rectangular geometries is presented. Also, the traditional finite element formulation is modified to allow the application of essential boundary conditions without having to constrain the nodal values of the tangential derivative of the temperature. The resulting higher order element and modified FEM formulation are used to solve an example problem and the accuracy of the solution is compared with solutions obtained using the traditional linear, quadratic, and cubic Serendipity elements to show the efficiency, in terms of accuracy per number of degrees of freedom, of the proposed approach for finding the nodal values of the temperature gradients, which are required to compute the nodal values of the heat flux vector.

Using phase field modeling simulation approach we investigate the effect of various parameters on the primary and secondary dendrite arm spacing during directional solidification in a single component system. In previous studies the effect of temperature gradient was assumed to be negligible in the transversal directions with a temperature rate equal to the product of thermal gradient and solidification rate. In our study the temperature field is obtained from energy conservation equation by considering the balance of latent heat released in the regions where solidification occurs and energy dissipation due to directional temperature gradient as boundary condition. In our simulations, we implemented a numerical method that enables the investigation of solidification in larger domains. Specifically, the temperature and the order parameter equations are solved only in the domains close to the solidification front; approach that reduces the computational costs significantly. We investigate the interplay and the effect of thermal gradient, solidification rate, undercooling temperature, and the cooling heat flux on arm spacing. By using a well-established power law relation the primary and secondary arm spacing are calculated for various solidification parameters. We also show that, for large heat fluxes, the secondary arm spacing is almost constant for different undercooling temperatures; behavior that demonstrates the need for correction of the power law relation by including the effect of heat flux.

Design processes and analytical modeling are presented showing creation of a low-cost concentrating photovoltaic-thermoelectric (PV/TE) hybrid power system for research and laboratory teaching built using a small upcycled satellite dish. Today, concentrated solar hybrid PV/TE systems are drawing significant research attention and funding investment. However, the literature lacks examples of how this cutting-edge energy technology can be made accessible at low cost for STEAEM education at universities, vocational institutions, and high schools. By applying Energy Engineering Laboratory Module ( EELM™ ) design principles and pedagogy, a process is presented to make this technology easily accessible at low cost. The concentrating solar hybrid PV/TE system presented here is divided into four subsystems: 1) a concentrator, 2) a PV/TE generator, 3) data acquisition, and 4) a cooling system. The key engineering decisions governing the design for each sub-system are described. In addition, a thermodynamic analysis is presented to predict the on-sun steady-state temperature profile of the PV/TE generator at the focus of the concentrator and to determine how much electrical power it will produce. The concentrator used is a salvaged miniature satellite dish, which is coated with mirrored tape to reflect sunlight upon a focal point. Scavenged at no cost, the satellite dish is a sectioned paraboloid of rotation offset from the vertex and the axis of symmetry. However, which paraboloid section the dish represents is unknown. A technique is presented to find the focal point and to use this information to correctly position a shadow-casting gnomon to ensure proper on-sun alignment. A method to experimentally confirm the focal location and size the PV is also provided. A key research question for solar concentrating hybrid PV/TE power systems at this size scale is whether it is better to actively cool the TE cold side via forced convection or simply allow cooling via natural convection. The thermodynamic heat balance analysis presented to address this question finds that while forced convection does better cool the PV module, increasing its efficiency and power output, the parasitic energy expenditure of the cooling fan far exceeds the additional power produced. It is therefore more beneficial to rely on natural convection on the TE cold side to maximize power production of the overall PV/TE module. Two experimental apparatuses were built consisting of a PV module backed by TE generators and instrumented with thermocouples to determine the internal temperature gradient while multi-meters read steady-state PV and TE power output. A halogen lamp placed at various distances from this array approximates concentrated sunlight, which is measured via pyranometer. These experiments validate conclusions drawn from the theoretical model.