Bulk density measurements of radioactive materials can give significant information about radiation-induced compositional and structural changes. Traditionally, density measurements of solids are performed using hydrostatic immersion techniques based on Archimedes‘ principal, the precision of which is highly dependent on knowing the density profile of the immersion fluid. However, radioactive materials are inherently self-heating, producing thermal gradients within the immersion fluid, which induce convective flow and density gradients around the sample. In this study, COMSOL Multiphysics models were generated to study how the immersion fluid thermo-physical properties, sample size, shape and orientation, and heating power output affect the buoyancy force on the sample and ultimately the precision of the density measurements. A dimensionless equation was developed and will be fit to simulation and experimental data to build a correlation between the thermo-physical properties of the immersion fluid and the variance in the resulting buoyancy force on the samples with varying heat outputs. Thermal models will be validated experimentally by immersing self-heating metal mimics — tungsten samples with embedded surface mount resistors connected to a controllable power source — into various immersion fluid baths. Schlieren and laser-induced fluorescence flow visualization techniques around the self-heating mimics can be used to validate the density profiles predicted by the numerical models. Only the beginning setup of Schlieren experiments will be discussed as they are ongoing. This correlation, along with concurrent chemical compatibility studies not discussed in this publication, will help derive fluid selection parameters to increase the precision of density measurements. The validated numerical models can also be used to improve density measurement setups and understand where data probes, e.g., thermocouples, should be located to realize the density profile in the immersion fluid during sample measurement.

The lid-driven flow inside a porous square cavity is numerically simulated. The porous media is modelled on the microscopic scale (heterogeneous porous medium) with a square heat conductive single block representing the solid constituent. Conversely, the fluid relies between the block and the cavity surfaces. A vertical positive thermal gradient, obtained by keeping the sliding-lid temperature T H higher than the base one T C , aligned with the gravity force enables a gravitational stable condition where the buoyant-induced flow does not occurs spontaneously. Instead, the flow comes about as the cavity top surface slides with constant velocity. Conservation equations are applied separately for each constituent and are coupled by boundary conditions at the fluid to solid interface (block surface). The Boussinesq-Oberbeck approximation accounts for the buoyant effects. The equations are solved via the finite volume method with the use of the SIMPLE algorithm for the pressure-velocity coupling and QUICK interpolation scheme for the treatment of the advection terms. The aim of the present work is to investigate how variations on the flow parameters and the block size affect the thermal process throughout the cavity. A top lid velocity based Reynolds number evaluates the intensity of the forced convection process while the Grashof number is associated with the intensity of buoyancy. The flow parameters cover only the laminar regime, such as 10 2 ≤ Re ≤10 3 and 10 3 ≤ Gr ≤10 7 . The Re and the Gr numbers are also analyzed by the means of the Richardson number, Ri , which accounts the relative predominance of buoyancy over the inertia effects. Moreover, a clear fluid cavity and enclosure configurations with three different block dimensions, namely B = 0.3, 0.6 and 0.9, are simulated. The heat transfer across the cavity can be characterized as a competitive effect, since the flow is hindered as the buoyancy effect rises. Results show that an increase in Re , or decrease in Gr , enhances the heat transfer, revealing a convection dominant regime. Alternatively, an increase in Gr , or a decrease in Re , leads the fluid to a stagnant-prone condition where a conduction dominant regime is verified. Thus, the surface-average Nusselt number, Nu av , tends to unity as the flow is confined to the adjacency of the sliding-lid. The placement of the single block in the cavity can enhance or hinder the heat transferred, depending on the flow regime. For instance, if a B = 0.6 block is inserted in the presence of a convection dominant regime, the Nu av is increased. Conversely, if the fluid is quiescent, a B = 0.6 block alters the flow path and the Nu av decreases. Intense blockage effects are observed for larger values of B since the block interferes on the flow more significantly. For a convection dominant regime, for instance, a B = 0.9 block causes the Nu av to drop. However, in the presence of stagnant fluid, the same obstacle forces the flow to circumvent it. Thus, the Nu av number increases, indicating that heat transfer mode returns to a convective pattern.

A three-domain conjugation heat transfer problem, the model of a phase-change shell-and-tubes heat exchanger, was studied both experimentally and numerically. The saturated water was impinged onto an array of hot tubes that arranged in a confined cylindrical cavity, and then boiled during the travelling to the downstream. The tubes were hollow, through which the hot air was passing trough and supplied the heat to boil the water on the shell side. Therefore, the problem involved three conjugated heat transfer domains: outside-tube two-phase boiling flow, solid conduction in tubes and inside-tube single-phase convective flow. In the experiments, the heat transfer at three different water inlet velocities and nine different air inlet temperatures were tested. The measured wall temperatures and the transferred energy were found to be independent of water inlet velocity. The wall temperatures in the core area (in the vicinity of the dense tube array) were relatively uniform. However, a huge temperature gradient was measured at the peripheral region. The experimental data were used to validate the simulation that accomplished by ANSYS-Fluent V16.1. High quality hexahedral meshes were generated and the PRI boiling model was adopted to simulate the boiling flow and heat transfer. It showed that the simulation results agreed well with the experimental data. Based on the simulations, the detailed fluid field and the wall temperature distributions were analysis and discussed.

Confined bubbly flows in millimeter-scale channels produce significant heat transfer enhancement when compared to single-phase flows. This enhancement has been demonstrated in experimental studies, and some of these studies conclude that the enhancement persists even in the absence of active nucleation sites and bubble growth. This observation leads to the hypothesis that the enhancement is driven by a convective phenomenon in the liquid phase around the bubble instead of sourcing from microlayer evaporation or active nucleation. Presented here is a numerical investigation of flow structure and heat transfer due to a single bubble moving through a millimeter-scale channel in the absence of phase change. The simulation includes thermal boundary conditions designed to match those of a recent experiment. The channel is horizontal with a uniform-heat-generation upper boundary condition and an adiabatic lower boundary condition. The Lagrangian framework allows the simulation of a channel of arbitrary length using this smaller computational domain. The fluid phases are modeled using the Volume-of-Fluid method with full geometric reconstruction of the liquid/gas interface. The liquid around the bubble moves as a low-Reynolds-number unsteady laminar flow. In a square region from the trailing edge of the contact line to one nominal bubble diameter behind the bubble, the area-averaged Nusselt number is, at its greatest, 4.7 times the value produced by a single-phase flow. Bubble shape and speed compare well to observations from the recent experiment. The heat transfer enhancement can be attributed to flow structures created by bubble motion. Multiple regions have been observed and are differentiated by their respective vortex characteristics. The primary region exists directly behind the bubble and exhibits the highest enhancement in heat transfer. It contains channel-spanning vortices that move cold fluid along the centerline and edge of the vortices from near the far wall of the channel to the heated wall. The cold fluid delivered by this motion tends to thin the thermal gradient region near the wall and directly behind the bubble and results in the highest local heat transfer coefficients. This vortex drives a bulk exchange of fluid across the channel and elongates the area of heat transfer enhancement to several bubble diameters. The secondary region is a set of vortices that exist to the side and slightly behind the bubble. These vortices rotate at a shallow angle to the primary flow direction and are weaker than those in the other regions.

Thermal efficiency of energy conversion systems such as gas turbines can be increased greatly with an increase in the turbine inlet temperature of combustion gases. However, this necessitates the use of efficient cooling techniques in addition to thermal barrier coatings (TBCs) to help significantly improve the life expectancy of gas turbine blades. The effect of TBC use is the formation of oxides, particularly alumina, at the interface of the ceramic top coat and bond coat material during in-service application. This effect is well known to cause failure of TBCs exposed to extreme high temperature environments. The objective of this paper is to present a micro-scale finite difference thermal model for the TBC-Substrate system that considers growth of the TGO layer and predicts in-situ thermal gradients. The governing equation is the transient heat diffusion equation discretized over a 1-D domain using mean value finite volume method with grid adaptation for zones involving depletion of bond coat (BC) material and TGO growth; hence, necessitating a moving interfacial boundary problem. The resulting algebraic equations are simultaneously solved in MATLAB to produce temperature distributions and BC/TGO interfacial locations. The model has utility in studying the evolution of residual stresses and hence prediction of TBC durability and failure.

The requirement that the propellants used in launch vehicle systems must also provide the thermal energy to be converted to kinetic energy in the rocket nozzle has plagued rocket designers since the dawn of the space age. Beamed propulsion systems, however, avoid this constraint by placing the energy source on the ground and transmitting the energy to the spacecraft via microwaves. This computational work evaluates three different heat exchanger channel designs for use in a beam propulsion spacecraft. The working fluid was hydrogen and the input energy was 1.3 kW. The increase in axial temperature along the 0.1 m long channel was as high as 2000 K. In addition, it was found that despite the very small diameter of the minichannels, 1 mm, each design produced extreme temperature gradients across the channel cross section. These temperature gradients affected the velocity profile; the maximum velocity was not located at the channel center.

The thermal efficiency of a gas turbine is largely dependent on the turbine inlet temperature (TIT). Modern gas turbines may operate at temperatures as high as 2000K, which is higher than the melting point of the material in use. Hence, thermal protection of gas turbine hot components is a big challenge. Film cooling is the most common cooling technique adopted in this application. In film cooling, coolant is injected at discrete locations along the metal surface, which forms a layer of cool air immediately over the hot surface, thus, protecting it from direct contact with hot mainstream air. The cooling is strong along the centerline of the hole in the downstream region and rapidly decreases over the span-wise direction. The distributed cooling can result in large thermal gradients, inducing thermal stresses in the material. The region with least cooling may lead to a cascade failure of the blade. Film cooling with backward injection holes has been proved to reduce this effect. In the current work, backward coolant injection scheme is explored under fan-shaped holes numerically. Fluent, a commercial CFD software, is used in the current work for numerical simulations. Effects of blowing ratio, injection angle, and turbulence are considered. Numerical results show that fan-shaped holes are better than simple cylindrical holes in terms of both cooling effectiveness and its uniformity. Numerical results are validated with experimental results. The image of temperature fields on cooling surface is captured with an Infrared camera.

Void formation is encountered in the form of air pockets during preparation of composite thermal energy storage systems, consisting of phase change materials (PCM) infiltrated into a high-conductivity porous structure. The presence of voids within the pores of a porous structure degrades the thermal and phase change behavior of such composites. Recent work devoted to multiphase modeling of the infiltration of PCM in liquid state into porous media and formation of voids showed that among the various contributing driving forces (i.e. gravity, pressure gradient and interfacial forces), the interfacial forces (resulting from surface tension and contact angle) play a significant role at the pore level. Additionally, modeling the solidification and melting of PCM within the pores in presence of a void revealed that there is a temperature gradient along the interface between the PCM and void. Considering the surface tension as the major driving force at the pore level, this temperature gradient is large enough to give rise to a gradient in surface tension that then triggers the Marangoni convection at the interface. Thus, as a convection mechanism, it affects the phase change process as well as the interface shape. Therefore, in this paper, the effects of the Marangoni convection on PCM solidification time and shape of the interface was investigated at the pore level. A numerical approach was employed for solidification of a PCM based on the combination of the Volume-of-fluid (VOF) and enthalpy-porosity methods, including the variation of the surface tension with temperature, i.e. Marangoni effects. A two-dimensional model of a pore was developed based on the average geometric features of the pores in a porous structure with interconnecting pores. Following the grid independence study, the transient simulation of solidification was performed, whereas the PCM within the pore and the air within the void were treated as incompressible liquid and compressible gas, respectively. The liquid density change during the solidification was included to explicate the formation of shrinkage void and its distribution within the pores. The PCM solidification time and shape of the final interface between the PCM and air pocket (representing the amount and distribution of the shrinkage void evolving during the solidification) were extracted and compared between the cases with and without Marangoni convection. For verification purposes, the volume of the predicted infiltration void is in agreement with experimental measurements and the volume of the shrinkage void shows a good agreement with theoretical volume change. The final shape of the interface was justified and turned out to be in agreement with the prevailing Marangoni convection pattern.

LASER surgery on the human eye is intended to reduce a person’s dependency on glasses or contact lenses. Any type of Laser surgery has heat effects on the eye. In laser surgery specific parts of the eye are exposed to concentrated high heat doses, too high heat at a certain spot results in permanent medical damage to the specific exposed eye cells. Precise temperature monitoring of the live interior of the human eye is not possible with the current technology. Published modeling assumes that the human eyeball is at a constant temperature, mostly at 37 °C. Understanding the exact temperature gradients in the prepared open human eyeball in room temperature before surgery is a first step in better understanding the heat effects of either laser surgery on specific treated spots of the cornea, or the effects of insertion of synthetic lenses in the human eye, or treating the retina with laser. In this article the anatomy of the human eyeball, dimensions, and properties are considered in constructing a finite element steady state thermal model of the normal open human eye for an adult, in preparation for surgery under normal room conditions. Also, room boundary conditions are used. Based on the model, the temperature gradients in the open eye are reported.

Flat heat pipes (heat spreaders) are similar to cylindrical heat pipes. But they have received significant attention recently because of their advantages over conventional cylindrical heat pipes with regard to their large surface area, isothermal heat delivery, geometry fit. For example, Thermal Ground Planes (TGPs) are flat, thin (less than 3 mm thick) heat pipes which utilize phase change cooling. The goal is to use TGP’s as universal heat spreaders in microelectronic cooling applications. These TGPs will act as a new generation of high-performance, integrated systems to work at a high power density without difficulties from temperature gradients, increased weight, or extra complexity. In addition to being able to dissipate high thermal powers, they have very high effective thermal conductivities and can operate in high adverse gravitational fields due to nano-porous wicks. This work shows the effect of vapor pressure, wall shear stress and the interfacial shear stress in the liquid pressure of the flat heat pipes and a comparison with CFD results. Also, this paper offers a design for flat heat pipe charts that avoids the effects of vapor pressure, wall shear stress and the wick-vapor interface to the liquid pressure for most well-known working fluids.

Tripod hole film cooling designs can provide improved cooling with reduced coolant usage. It is essential to determine the impact of strong cooling near hole locations due to increased thermal gradients around the hole region. In this paper, efforts have been made to predict the same using a numerical analysis. Local temperature distribution on the blade surface causes different rates of expansion, resulting in a differential strain, furthering the impact of the numerical study and creating a need to understand the thermo-mechanical behavior of blade design. The thermal stresses that are generated near the film cooling holes are compared for different cooling hole shapes and eventually weigh the thermal advantage of a tripod hole over the cylindrical standard design. Standard k-ε was used to validate the CFD results, ensuring by a fine mesh of 5 million tetrahedrons. Validation includes comparison of the experimental results, with a numerical one where similar blade material and working temperatures are used. Upon validation, CFD was run again at engine temperature conditions with Haynes230 alloy as the blade material. Surface temperature contour enabled structural analysis using a finite element approach. Equivalent stresses (Von-Mises stresses) on the blade emerging from the analysis provided more information about stress dependency on blowing ratio.

Advancements in thermal interface materials have reached the resolution limit of typical ASTM D5470 based testers, which is around ±1×10 −6 m 2 K/W. As the interfacial resistance is reduced, the temperature difference at the interface decreases and ultimately becomes difficult to measure. Standard ASTM testers utilize precise temperature sensors and knowledge of the thermal conductivity of the heat meter bar to resolve the temperature difference at the interface. It is difficult to resolve interface resistances on the order of 1×10 −6 m 2 K/W, even when precision RTDs with a resolution of ±0.001°C are utilized, as the location uncertainty of the sensor can become important. Increasing the temperature difference across the interface is necessary for further improvement in the resolution. This work presents a miniature ASTM type tester that was developed to address the resolution limits of standard testers by reducing the heat meter bar thickness, using a chip resistor as the heater element, and using an IR camera to measure the temperature gradient along the meter bar. Reducing the length of the heat meter bars reduces the overall resistance, and increases the resistance of the interface relative to that of the meter bars. Because of the reduced size scale of the miniature tester, measurement of the temperature profile using the typical ASTM approach of embedding temperature probes along the length of the meter bar was not feasible but instead was achieved using a relatively inexpensive uncooled long wavelength infrared camera with a microscope attachment in order to focus down to ∼100 microns. Although the IR camera increases the uncertainty of the measured temperatures, this method is shown to measure a thermal interfacial resistance of 1.45×10 −6 m 2 K/W with an uncertainty of ±1.1×10 −7 m 2 K/W, where the sample interface was a 2 mil AuSn preform soldered interface.

Although the experimental capability to design and fabricate specific microstructures in materials has advanced significantly in the past few decades, a description of their effective thermal behavior on the macroscale has not kept pace. The length scales of the fundamental processes (approximately 100 nm–10 micron, or microscale) are typically several orders of magnitude smaller than the scales dictated by a given application (1 mm–1 cm, or macroscale). We are interested in extending classical asymptotic approaches to allow for the spatial pattern wavenumber to vary on the macroscale variables and to find how changes in microstructure geometry affect macroscopic properties and transport. To this end, we consider here the thermal transport of a binary fluid (coolant) through nonuniformly spaced laminates, as a simple model for heat sinks in electronics. Power is continuously being generated by the laminates, and the local rates of heat transport depend on the composition of the binary mixture. However, segregation of the mixture can occur under favorable temperature gradients, which could potentially affect the local heat transfer from the laminate to the coolant. We derive sets of effective equations that describe the evolution of the temperature field, the local coolant composition, the fluid velocity and the pressure over the macroscale that are driven by the net effects of the microscale processes. The evolution of these quantities is driven by the local heat generation within the laminates, lamellar geometry gradients, the prescribed flow rate of coolant entering the system, along with the fundamental material properties of the laminates and the coolants. Microscale values of all of these quantities are known in terms of the solutions to these effective eqautions. We are especially interested in geometries in the laminate spacing which allow for better thermal transport by the coolant for a prescribed power distribution.

Jet flow phenomenon is important in enhanced nucleate boiling heat transfer processes and applications. When heater sizes scale down, jet flow can be observed due to the Marangoni convection around bubbles staying on microscale heated surface. In this paper, two fluids were employed for comparing and demonstrating the effect of Marangoni convection on bubble behaviors on micro heating wire. One was ultrapure water and the other was aqueous n-butanol solution, a self-rewetting fluid. Bubble-top jet flow for water and multi-jet flows for n-butanol solution were observed around a platinum micro heating wire by high speed CCD camera. Corresponding numerical simulation proved that it is the Marangoni convection that attracts the sub-cooled water to flow from the super-heated microlayer at the bottom to the top of a stationary bubble. For n-butanol solution, however, the Marangoni convection can induce it to flow oppositely, which causes the subcooled solution to flow onto the heated surface. The simulation for the solution is in good agreement with the experiment where the subcooled liquid nears the bubble-top flow towards the bottom of bubble or the heated surface and hence the multi-jet flows occur. The multi-jet flows can sustain for a long period and cause transient chaos at the super-heated thin liquid layer near the heated surface. The temperature around the bubble presented sharp temperature gradient and the velocity in the near-wall region is almost vertical to the wall. The experimental and numerical studies on the effect of surface tension and thus Marangoni convection are crucial to the mechanisms of subcooled nucleate boiling of fluids.

The study investigates the temperature gradients achieved across a thermoelectric generator by using the exhaust gases from a vehicle as a heat source and the radiator coolant as the cold sink. Various heat transfer enhancement features are employed in order to achieve as high a temperature gradient as possible. Effect of flow Reynolds numbers and inlet temperatures are examined to create a body of data predicting total power output from the TEG. Data is normalized against results from baseline heat exchanger designs investigated in the past. The experiments are carried out at 1/5th scale of the previously examined geometry. Impingement geometry is employed on the coolant side to enhance heat transfer. The experimental test sections are fabricated using metal 3D printing. Water is used instead of radiator coolant and heated air is used for exhaust gases. The results from the experiments provide valuable data which can be used for system level optimization.

Thermal ground planes (TGPs) are flat, thin (external thickness of 2 mm) heat pipes which utilize two-phase cooling. The goal is to utilize TGPs as thermal spreaders in a variety of microelectronic cooling applications. TGPs are novel high-performance, integrated systems able to operate at a high power density with a reduced weight and temperature gradient. In addition to being able to dissipate large amounts of heat, they have very high effective axial thermal conductivities and (because of nano-porous wicks) can operate in high adverse gravitational fields. A three-dimensional (3D) finite element model is used to predict the thermal performance of the TGP. The 3D thermal model predicts the temperature field in the TGP, the effective axial thermal conductivity, and the evaporation and the condensation rates. A key feature of this model is that it relies on empirical interfacial heat transfer coefficient data to very accurately model the interfacial energy balance at the vapor-liquid saturated wick interface. Wick samples for a TGP are tested in an experimental setup to measure the interfacial heat transfer coefficient. Then the experimental heat transfer coefficient data are used for the interfacial energy balance. Another key feature of this model is that it demonstrates that for the Jakob numbers of interest, the thermal and flow fields can be decoupled except at the vapor-liquid saturated wick interface. This model can be used to predict the performance of a TGP for different geometries and implementation structures. This paper will describe the model and how it incorporates empirical interfacial heat transfer coefficient data. It will then show theoretical predictions for the thermal performance of TGP’s, and compare with experimental results.

The electrocaloric effect (ECE) refers to the change in temperature and/or entropy of a dielectric material due to the electric field induced change of dipolar states. Giant ECE is discovered in P(VDF-TrFE) ferroelectric copolymers near ferroelectric-paraelectric (F-P) transition temperature which is normally much higher than room temperature. This paper presents the two defect-inducing methods to lower and broaden working temperature range of P(VDF-TrFE) based copolymers for ECE, and thus make it preferable for practical cooling device. Giant ECE is experimentally demonstrated in large temperature range (0–55°C). In addition, an electrocaloric oscillatory refrigerator (ECOR) was proposed and simulated by finite volume method and its high performance was theoretically demonstrated. Temperature gradient larger than 30 °C can be maintained across the two sides of a 1 cm device. For ΔT = 20 °C cooling condition, a high cooling power (5.4 W/cm 2 ) and significantly higher coefficient of performance (COP) can be achieved (50% of Carnot efficiency).

Solar thermophotovoltaic (STPV) systems convert solar energy into electricity via thermally radiated photons at tailored wavelength to increase energy conversion efficiency. In this work we report the design and analysis of a STPV using a high-fidelity 2D axisymmetric thermal-electrical hybrid model that includes thermal coupling between the absorber/emitter/PV cell and accounts for non-idealities such as temperature gradients and parasitic thermal losses. The radiative spectra of the absorber and emitter are engineered by using two-dimensional periodic square array of cylindrical holes on a tantalum (Ta) substrate. The optimal solar concentration and resulting temperature are determined by considering the energy losses associated with re-emission at the absorber, low energy (below band gap) emission at the emitter, and carrier thermalization/recombination in the PV cell. The modeling results suggest that the overall efficiency of a realistic planar STPV consisting of Ta PhCs and existing InGaAsSb PV cells with a filter can be as high as ∼8%. The use of high performance PhCs allows us to simplify the system layout and operate STPVs at a significantly lower optical concentration level and operating temperature compared with STPVs using metallic cavity receivers. This work shows the importance of photon engineering for the development of high efficiency STPVs and offers design guidelines for both the PhC absorber/emitter and the overall system.

Cooling poses as a major challenge in the IT industry because recent trends have led to more compact and energy intensive microprocessors. Typically microprocessors in current consumer devices and state-of-the-art data centers are cooled using relatively bulky air cooled heat sinks. The large size heat sinks are required due to the poor thermophysical properties of air. In order to compensate for the poor thermal properties of air, it is typical to use chillers to pre-cool the air below the ambient temperature before feeding it to the heat sinks. Operating the chillers requires additional power input thereby making the cooling process more expensive. The growing cooling demand of electronic components will, however, render these cooling techniques insufficient. Direct application of liquid-cooling on chip level using directly attached manifold microchannel heat sinks reduces conductive and convective resistances, resulting in the reduction of the thermal gradient needed to remove heat. Water is an inexpensive, nontoxic and widely available liquid coolant. Therefore, switching from air to water as coolant enables a much higher coolant inlet temperature without in any way compromising the cooling performance. In addition, it eliminates the need for chillers and allows the thermal energy to be reused. All these improvements lead to higher thermal efficiency and open up the possibility to perform electronic cooling with higher exergetic efficiency. The current work explores this concept using measurements and exergetic analyses of a manifold microchannel heat sink and a small scale, first of its kind, hot water cooled data center prototype. Through the measurements on the heat sink, it is demonstrated that the heat load in the state-of-the-art microprocessor chips can be removed using hot water with inlet temperature of 60°C. Using hot water as coolant results in high coolant exergy content at the heat sink outlet. This facilitates recovering the energy typically wasted as heat in data centers, and can therefore result in data centers with minimal carbon footprint. The measurements on both the heat sink and the data center prototype strongly attest to this concept. Reuse strategies such as space heating and adsorption based refrigeration were tested as potential means to use the waste heat from data centers in different climates. Application-specific definitions of the value of waste heat were formulated as economic measures to evaluate potential benefits of various reuse strategies.

This paper performs molecular dynamics simulations on flow and heat transfer process of nanofluids containing spherical nanoparticles with various diameters (2–6 nm). Instantaneous rotational velocity components of nanoparticles in a flow field with and without a temperature difference are outputted and compared. Number density method is used to examine the thickness of absorption layer. And by equally dividing the fluid into 60 fluid layers, temperature distributions of nanofluids and base fluid are examined. It was found that rotational speed of nanoparticle decreases with an increasing diameter. By applying temperature difference rotational speed of nanoparticles are generally increased. The rotational speeds of nanoparticles are generally about 1E9 rad/s. the rotation of nanoparticles is attributed to Brownian motion due to their nanoscale size. The diameter of nanoparticles has little effect on the thickness of the absorption layer, and the thickness of absorption layer is about 0.8 nm. By comparing temperature distributions of nanofluids and base fluid, it was found that the internal temperature difference in nanofluids is less than that of base fluid. And according the temperature gradient in nanofluids near the solid wall will be larger, which is better for heat transfer. This phenomenon is attributed to the fast-rotating nanoparticles accompanied by the absorption layer of liquid atoms. The present work examines the rotation of nanoparticles and absorption layer, which is the basis of understanding heat transfer mechanism in nanofluids and proposing mathematical description for the transfer process.