Direct Heat Transfer Research Articles

Thermophotovoltaic power conversion using a superadiabatic radiant burner

A new configuration of a 5–10W thermophotovoltaic (TPV) device integrated with a porous superadiabatic radiant burner (SRB) is suggested and experimentally studied. The silicon carbide (SiC) SRB (emitter) consists of a small-pored upstream section (PM1) and a large-pored downstream section (PM2). PM1 is the section where the incoming fuel-air mixture is preheated internally and PM2 is the section where flame is established. Also, a separate preheater is attached on the SRB to externally recover heat from the exiting flue gas and preheat the inlet air for the burner, and radiation rods are embedded at the interface between the PM1 and PM2 to extract heat from the flame and transfer it to radiating disk surfaces. Radiation from the disk surface is used for the TPV power conversion, reaching gallium antimonide photovoltaic cells (PVCs) with proper quantum efficiencies (up to 80%) through a quartz plate for preventing direct convectional heat transfer from the exhaust gas onto the PVCs. Under optimized conditions, uniform radiation provides adequate TPV performance, particularly indicating reasonable emitter efficiencies (up to 32%) with the enhanced disk temperature even for fuel-lean condition. Thus, the present configuration of the SRB-integrated TPV device can be used in practical applications, avoiding high-level noise without any moving parts.

Crowdsourcing Urban Air Temperatures through Smartphone Battery Temperatures in São Paulo, Brazil

AbstractCrowdsourcing as a method to obtain and apply vast datasets is rapidly becoming prominent in meteorology, especially for urban areas where routine weather observations are scarce. Previous studies showed that smartphone battery temperature readings can be used to estimate the daily and citywide air temperature via a direct heat transfer model. This work extends model estimates by studying smaller temporal and spatial scales. The study finds the number of battery readings influences the accuracy of temperature retrievals. Optimal results are achieved for 700 or more retrievals. An extensive dataset of over 10 million battery temperature readings for estimating hourly and daily air temperatures is available for São Paulo, Brazil. The air temperature estimates are validated with measurements from a WMO station, an Urban Flux Network site, and data from seven citizen weather stations. Daily temperature estimates are good (coefficient of determination ρ2 of 86%), and the study shows they improve by optimizing model parameters for neighborhood scales (<1 km2) as categorized in local climate zones (LCZs). Temperature differences between LCZs can be distinguished from smartphone battery temperatures. When validating the model for hourly temperature estimates, the model requires a diurnally varying parameter function in the heat transfer model rather than one fixed value for the entire day. The results show the potential of large crowdsourced datasets in meteorological studies, and the value of smartphones as a measuring platform when routine observations are lacking.

Free convective energy management of an inclined enclosure mounted with triple heating elements: Multiple morphology optimizations with unique global energy supply

An objective-oriented optimization procedure consisting of a simplified conjugated gradient methodology and a two-dimensional fluid and thermal energy transfer model is implemented to discover optimal morphologies of local heating elements. Direct heat transfer problem and inverse optimization problem are subsequently investigated. Full simulation shows that thermal Rayleigh number, enclosure inclination, heating strength ratio and size ratio of local heating sources have significant effects on the natural convection heat transfer in the inclined enclosure, asymptotically modeling like solar energy collectors or electronic boxes. The fluid flow and energy transfer inside the enclosure are analyzed in some representative situations, by the simultaneous use of streamlines, isotherms and heatlines. Inverse natural convection solutions on the maximization of global conductance are addressed, concerning on the effects of thermal Rayleigh number, inclination angle, heater strength ratio and heater length ratio. Mathematical correlations have been proposed by the multiple linear regressions to identify the role of governing parameters on maximizing global conductance and optimal morphologies of the discrete heat sources, concerning on the unique global heating flux. Present numerical methodology and inverse procedures could benefit free cooling of electronic components and effective solar collection elements.

Experimental studies on high-pressure high-temperature contact-condensation at falling jets in the TOPFLOW pressure-tank

Abstract We report on generic experimental studies dealing with the direct contact condensation of steam at a falling sub-cooled water jet at high pressure and temperature. This generic problem concerns some safety-relevant thermal hydraulic scenarios in light water reactors. One of such is the question for heat transfer and mixing when a sub-cooled jet of water in an emergency core-cooling scenario enters a hot pressurized component of the primary reactor circuit. The involved phenomena of heat transfer are complex in this case, reaching from direct condensation of steam via steam bubble entrainment to single phase mixing of hot and cold water. Experiments were performed in the TOPFLOW pressure tank, which is an experimental facility for high-pressure thermal hydraulics experiments in pressure equilibrium. The facility has been designed for studying steam-water two-phase flows at pressures of up to 50 bar. It enables to run experiments in flow domains of complex shape without high difference pressures across the wall. The concept therefore allows us to use thin metal walls and even glass windows to observe flows in complex geometry domains with the help of video camera and to considerably reduce cost and complexity of experimental installations. The experimental results show, that high ambient pressure promotes the direct contact condensation heat transfer of subcooled jet flow heavily.

Experimental validation of heat/mass transfer analogy for two-dimensional laminar and turbulent boundary layers

Heat and mass transfer from a solid surface into a fluid stream are both diffusion driven processes. Their constant property mathematical model equations are identical for identical flow and equivalent boundary conditions, when Lewis number is equal to unity. Problems with engineering applications can be transformed from one domain to the other for practical and economic advantages. Mass transfer measurement using naphthalene sublimation can be faster, of higher resolution, economical, and more accurate than a direct heat transfer measurement. The diffusion rates by heat in air and naphthalene in air are quantitatively different. Hence, the advantages of naphthalene sublimation measurement are justified when the heat/mass transfer analogy is experimentally and/or analytically verified and an analogy factor (F=Nu/Sh) is determined.Heat/mass transfer analogy is experimentally verified in this study for two dimensional laminar and turbulent boundary layer flows over a flat plate. A thermal boundary layer technique is used to measure local heat transfer coefficient (Nu) and a naphthalene sublimation measurement is used to evaluate local mass transfer coefficient (Sh) for two similar plates subjected to the same boundary layer flow. The heat/mass transfer analogy factor is calculated using the measured Nu and Sh. It is found to agree with the analytical prediction of 0.677 within 2% for the laminar case and is found to be constant at 0.667 for the turbulent flow.

Cladding-free efficiently tunable nanobeam cavity with nanotentacles.

We propose a cladding-free, efficiently tunable, high-quality factor (Q) nanobeam cavity with subwavelength-period nanotentacles (NT), adequately investigate the performance of the cavity, and study the directional heat transfer. By virtue of the excellent heat transfer of Si nanotentacles, a tuning range of more than 6 nm wavelength, with 24mW and 10 KHz switching rate, and 13 μs raising time is experimentally obtained. This result is about twentyfold better than the previous work by Fegadolli [ACS Photon. 2, 470-474 (2015)]. A potential 12nm tuning range with identical power is also theoretically suggested by modifying the silicon structure. With an optimized design, these nanotentacles are demonstrated to have a minimal effect to the cavity and are available to serve as photonic waveguides. This cladding-free design, with a simple fabrication process, is comparable to other proposals in which deep etching, suspended treatment, and troublesome heterogeneous-integration may be needed. Finally but importantly, this smart design can be applied to other photonic cavities, particularly cavities such as ring/disk resonators, in which we reasonably predict a better tuning efficiency due to the thermal circulation. We believe this design is fairly suitable for applications in which light-matter interaction is of primary importance, such as sensing, particle trapping, cavity quantum electrodynamics (CQED), and III-V/Si hybrid lasers with external cavities.

Safety Analysis Embedded in Heat Exchanger Network Synthesis

Optimization of Heat Exchanger Networks (HEN) has received considerable attention in last decades, but a few studies on inherent safety. In this paper, risk assessment is considered simultaneously during the synthesis of HENs. As risks depend on the equipment selected, a superstructure enabling selection of direct and indirect heat transfer between hot and cold streams and different types of heat exchangers (HEs) was tested. The individual heat transfer and the overall HEN risk were analyzed. Different individual risk limits have been introduced for certain types of heat transfer, e.g. between two process streams or between utility and process streams. The sensitivity analyses were performed first, considering only toxicity as a risk, but later flammability and explosiveness were also simultaneously tested, in order to consider the most important aspects of safety. The results obtained indicate that rather significant changes in HEN designs can increase safety, while still exhibiting similar economic efficiency.

Synthesis of More Sustainable Total Site

Heat recovery between different streams of Total Site (TS) can significantly decrease external utility consumption. Decreased utility consumption leads to decreased impact on the environment resulting from lower GHG emissions produced by fuel. The optimal rate of heat recovery can be determined by establishing an appropriate trade-off between utility consumption and investment in heat transfer equipment. Therefore, it can be concluded that both economic and environmental pillars of sustainability encourage the goal of constructing TSs. However, the social pillar of sustainability should also be considered in order to ensure the overall sustainability of a TS. The social pillar is usually not included in the synthesis because different aspects of social performance are quite difficult to quantitatively determine. Safety is an important social aspect when considering TS, especially when TS complexes are located across areas with high population density where any failure is a potential source of events with catastrophic consequences. In order to construct a safe TS, one option is to perform risk assessment before the synthesis and forbid highly risky matches at the synthesis step or to perform the synthesis and the risk assessment simultaneously. In this study, risk assessment was performed simultaneously during the synthesis in order to design TSs with as low risks as are socially acceptable. The synthesis was performed in two steps. In Step 1 a globally optimal solution was obtained based on a simplified trade-off between investment and operating cost while simultaneously considering risk assessment using a mixed-integer linear programming (MILP) TransGen model, while in Step 2 a detailed synthesis considering risk assessment was performed on a reduced superstructure obtained in Step 1 with a mixed-integer nonlinear programming (MINLP) model called Total Site Synthesis model, which explicitly considers risk limits during optimization. The risk depends on the frequency of failures and the severity of the consequences. The former can be reduced by the selection of more suitable equipment and the latter by selecting indirect rather than direct heat transfer, selecting smaller sizes and safer operating conditions. The minimization of total annual costs (TAC) is a primary objective of this synthesis. There are significant differences in the results obtained when safety is not considered and when lower risk limits are set in order to obtain safer designs. It can be concluded that by performing TS synthesis using the proposed synthesis model, the inherent safety of the TS is significantly increased; however, this incurs economic expense.

Compact modeling approach for microchannel cooling and its validation

This paper presents a new compact modeling technique to describe the convective heat transfer realized by laminar flow of coolant in integrated microchannels used for thermal management of 3D ICs—with the aim of being able to implement this model for fast thermal simulators used in logi-thermal simulation frameworks. This model works only on laminar flow in straight channels. The compact model represents the convective heat transfer with a special resistor network representing one directional heat transfer realized by the flow of the coolant. The implementation of this model in a thermal field solver based on the successive node reduction (SUNRED) algorithm is also presented. The validation of the model is based on two examples. First, we compare the result obtained with the new model with the results provided by a commercially available CFD software for a simplified test case. The other pivot of the validation is an actual measurement of the thermal resistances of microfluidic cooling system under different flow-rates of coolant. The errors were calculated as the difference of the results of the direct implementation of the alternating resistors model, the modified SUNRED model and the result of a detailed CFD simulation for the test case. The error of the simulation of the cooling device is based on the difference of the directly measured and simulated thermal resistances. The comparison showed that the error of our new compact model was close to 5%.

Coherent long-range thermoelectrics in nonadiabatic driven quantum systems

We investigate direct energy and heat transfer between two distant sites of a triple quantum dot connected to reservoirs, where one of the edge dots is driven by an ac-gate voltage. We theoretically propose how to implement heat and cooling engines mediated by long range photoassisted transport. Additionally we propose a simple set up to heat up coherently the two reservoirs symmetrically and a mechanism to store energy in the closed system. The present proposals can be experimentally implemented and easily controlled by tunning the external parameters.

Comparison of inverse modelling and optimization-based methods in the heat flux estimation problem of an irradiative dryer/furnace

There are two major approaches in sequential (real-time) heat flux estimation problems using measured temperatures: (i) development of inverse heat transfer models that directly estimate heat flux and (ii) use of a combination of a direct heat transfer model (which estimates temperature using heat flux information) and an optimization algorithm. In physics-based solutions, using thermodynamics and heat transfer laws, the first approach is considered ill-posed and challenging, and the second approach is more popular. However, the use of artificial intelligence (AI) techniques has recently facilitated heat transfer inverse modelling, even for complex irradiative systems. Many of the claimed advantages of AI inverse models of irradiative systems result from the use of AI techniques rather than the inverse modelling approach. This research presents a rational comparison between the aforementioned approaches for an irradiative thermal system, both using AI techniques, for the first time. The results show that inverse models are superior because of their higher accuracy and shorter estimation delay time.

Optimum Orientation of a Mutually-Shaded Group of Buildings with Respect to External Solar Radiation

External, mainly solar, load can constitute up to 80% of the energy consumed by Heating, Ventilating and AirConditioning (HVAC) systems in buildings. Proportional attention is rarely given by designers to the optimum positioning or orientation of a building with respect to the sun so as to achieve minimal solar load. Even less attention has been given to groups of buildings located within a building complex for potential mutual shading. The general practice is to place them in a row or square formation. In this work, the optimum positioning of three high-rise buildings in close proximity to each other is investigated numerically with respect to solar radiation and potential mutual shading. Specifically, the effect of the relative locations of the buildings is tested with respect to solar radiation direction for a typical summer day in Kuwait City, Kuwait. The transient three-dimensional problem is solved using the Solar Load Model of the FluentTM finite volume computational fluid dynamics code. The solar load model calculates radiation effects from the sun's rays that enter a computational domain transiently based on the selected location. Specifically, the ray tracing approach in the model applies solar loads as heat sources in the energy equations. The solar calculator utility is used to construct the sun's location in the sky for the selected timeof-day, date, and position. As the circle containing the three buildings is rotated, the energy absorbed by the shaded one or two buildings changes significantly. Combined with air flow around the buildings, the interaction between convection and radiation heat transfer rates to the buildings can vary greatly. This variation should be taken into account when sizing HVAC equipment for the individual building. Typical TMY (Typical Meteorological Year) data for Kuwait City, Kuwait, is used to obtain boundary conditions for air velocity and temperature. Mass, momentum and energy conservation equations are solved in conjunction with the radiative Solar Load equations to obtain the combined effect. Results show that there exists an optimum orientation for the group for the selected locale and that the reduction in solar load for the optimum orientation for the group of three buildings and that the orientation effect on the total HVAC energy requirement (represented by the cooling load) is significant. The difference between the best and worst orientations was about 6%. Convection, even though non-negligible, has a somewhat smaller effect on the total heat transfer and thus cooling load. Ongoing work examines the effect of the sensitivity of energy savings to wind direction and convection heat transfer, by altering strength and direction of the breeze.

Performance analysis of a two-stage evaporative cooler

In the present study, a mathematical model of a two-stage evaporative cooling (TSEC) system is presented, which consisted of two direct evaporative coolers and a heat exchanger. In this system, the temperature of the inlet air into evaporative pads decreases through indirect heat exchange with the outlet air stream from the conditioned space, using an unmixed heat exchanger. The outlet air from the conditioned space is also cooled down, prior to cooling the inlet air, through a second evaporation process. The governing equations of the energy and mass are presented and solved simultaneously. The numerical results are then compared with the experimental data reported in the literature to establish confidence in the model. The sensitivity of the performance of the system to the temperature and humidity of the ambient air, efficiency of the direct evaporative pads and overall heat transfer coefficient of the heat exchanger were calculated. The results are discussed to find the parameters with more significant effects on the performance of the system. The sensitivity analysis reveals that further improvements to the performance of the system is possible through increasing of the overall heat transfer within the indirect heat exchanger and increasing of the direct evaporative cooling efficiency.

Study on Thermal and Optical Properties and Influence Factors of Aerogel Glazing Units

Aerogel glazing units (AGUs) is a high-performance building glazing which can effectively reduce energy consumption due to its thermal and optical performance. In this paper, the effect on thermal and optical properties of AGUs with different particle sizes and filling thicknesses were investigated. The results indicated that visible light transmittance and solar direct transmittance of AGUs palpably increased as the particle size increased from 0.41mm to 0.93mm, while it lightly changed as the particle size continued to increase from 0.93mm. The heat transfer coefficient decreased by about 15% when the particle size was reduced from 2.7mm to 0.41mm. In conclusion, the increase of the thickness of aerogel can effectively decrease visible light transmittance, solar direct transmittance and heat transfer coefficient. Finally, color rendering index(CRI)the AGUs with different particle sizes and filling thicknesses were investigated and AGUs show good color performance.

Forced convection on grey cast iron plate-fins: Prediction of the heat transfer coefficient

The main goal of the present work is to evaluate the convective heat transfer coefficient at the surface of grey cast iron plate-fins. A hybrid numerical/experimental approach was adopted, i.e., temperature was measured at selected points at the fin surface and an inverse problem technique based on optimization was used to obtain the heat transfer coefficients. The direct heat transfer problem was solved numerically using the finite volume method, whilst the optimization problem was resolved based on particle swarm optimization (PSO). Firstly, the temperature dependence is investigated by comparing uniform, linear and parabolic equations for the heat transfer coefficient. The hybrid approach was validated through an energy balance applied to the finned surface. The parametric study was performed by assessing the influence of the fin spacing and flow velocity on the convective heat transfer coefficient: the results indicate that the convective coefficient is enhanced with increasing Reynolds number and fin spacing. Finally, the experimental results for the Nusselt number in the parametric study were condensed into a single new empirical correlation with good accuracy.

Effect of Hypoeutectic Boron Additions on the Grain Size and Mechanical Properties of Ti-6Al-4V Manufactured with Powder Bed Electron Beam Additive Manufacturing

In additive manufacturing, microstructural control is feasible via processing parameter alteration. However, the window for parameter variation for certain materials, such as Ti-6Al-4V, is limited, and alternative methods must be employed to customize microstructures. Grain refinement and homogenization in cast titanium alloys has been demonstrated through the addition of hypoeutectic concentrations of boron. This work explores the influence of 0.00 wt.%, 0.25 wt.%, 0.50 wt.%, and 1.0 wt.% boron additions on the microstructure and bulk mechanical properties of Ti-6Al-4V samples fabricated in an Arcam A2 electron beam melting (EBM) system with commercial processing parameters for Ti-6Al-4V. Analyses of EBM fabricated Ti-6Al-4V + B indicate that the addition of 0.25–1.0 wt.% boron progressively refines the grain structure, and it improves hardness and elastic modulus. Despite a reduction in size, the β grain structure remained columnar as a result of directional heat transfer during EBM fabrication.

Directional heat transport through thermal reflection meta-device

Directional heat transfer may be hard to realize due to the fact that heat transfer is diffusive. In this paper, we try to take one step forward based on the transformation thermodynamics. A special structure and meta-device is proposed to “reflect” the heat flow directionally–just like the mirror to the light beam, in which the heat flow just one-time changes the direction rather than gradually changing the directions in isotropic materials. The benefits of such thermal reflection meta-device are discussed by comparing the corresponding thermal resistance with the same structures of isotropic materials. The proposed meta-device is verified to possess the low thermal resistance and high heat transfer ability with least energy loss, and can be made by nature-existing isotropic materials with specific structures.

Investigation of the thermal-hydraulic characteristics of supersonic steam injector

Steam injector (SI) is known as passive jet pump and heat exchanging device which operates without external power source or mechanical machineries. It utilizes direct contact condensation heat transfer between steam and water-jet as a driving mechanism of the operation and is capable of discharging subcooled water at higher pressure than the inlet fluids pressure. In addition, it has an excellent heat transfer capability, more than 1000 times that of shell and tube heat exchanger. In the present study, thermal-hydraulics characteristics of the water-jet-centered supersonic SI system were investigated from both experimental and analytical approaches. The SI body was manufactured with stainless steel equipped with overflow port. The water injection nozzle was designed with shaft-driving mechanism to freely adjust the axial location of the water nozzle and steam inlet cross sectional area. High pressure steam was supplied to the SI from once-through boiler, which is capable of supplying saturated steam at the maximum pressure of 0.63MPa for the current test facility. Water jet was injected at mass flow rate of 0.4–0.8kg/s. Pressure and temperature measurements were conducted at inlet and outlet of the steam injector system as well as at the overflow port to investigate the operation characteristics of the SI. Results showed the water-jet centered SI’s promising functionality as a passive coolant injection in view of its quick-start up, operable condition limits, discharge pressure and heat transfer capabilities at current inlet conditions. In addition, obtained experimental results were compared with analytical model to assess the predictive capability of discharge pressure value and reasonable agreement was obtained.

Study of directional control of heat transfer and flow control in the magnetohydrodynamic flow in cylindrical geometry

Two-dimensional laminar electrically conducting flow and its heat transfer is considered and the control of heat transfer in different directions is analysed using a class of high accuracy numerical scheme in curvilinear coordinate system. Numerical flow solutions with temperature fields were obtained for range of Reynolds number 10 ≤ Re ≤ 40, Prandtl number 0.065 ≤ Pr ≤ 7 and Interaction parameter 0 ≤ N ≤ 5. For weak magnetic fields, the drag coefficient increases by 37% when the field direction is aligned with that of flow, and when the field is directed perpendicular to flow direction, it drastically increases by 390% for Re=40. For stronger magnetic field strengths, the drag coefficient increases like square root of interaction parameter. When no field is applied the heat transfer takes place in the entire region of downstream, but when the magnetic field is switched on, the direction of applied magnetic field influences the heat transfer to take place in the selected direction of the downstream by forming plumes in those directions. In contrast to a reduction in mean Nusselt number due to aligned magnetic field, the heat transfer increases due to transverse magnetic field.

Usage of Cold Spraying of High-Heat-Capacity Coatings for the Increase of Low- <inline-formula> <tex-math notation="LaTeX">$T_{c}$</tex-math> </inline-formula> Superconductors Stability

The thermal stability of NbTi Rutherford-type cables was improved with cold spray technology, namely, by depositing high-heat-capacity coatings onto the surface of the superconductors. Two short samples of identical NbTi Rutherford-type cables with different metallic/ceramic coatings were prepared and tested. The spray particles were deposited onto the cables made of eight monofilamentary NbTi/Cu wires via supersonic velocity impact. The temperature of the process is much lower than the melting point of the sprayed material. The doped sample contained 1 vol.% of large-heat-capacity substance (LHCS) Gd2O3, whereas the comparison sample did not contain any LHCS. The average heat capacity of the doped sample at 4.2 K was increased more than two times. During the tests, both samples at LHe temperature were subjected to short (∼1 ms) electromagnetic disturbances in the transverse constant magnetic field. It was found that the minimum quench energies for the sample with Gd2O3 were 1.4–1.5 times higher than that for the comparison cable without LHCS. The described method is very promising for the stabilization of dense massive windings where the direct heat transfer to the LHe is hampered and the superconductor operates in almost adiabatic conditions.