Heat exchanger performance data commonly contain redundant heat transfer rate measurements. Due to measurement uncertainties involved in the experiments, these redundant heat transfer rates have some discrepancies. While it is a common practice and adopted by engineering standards to use the arithmetic mean of heat transfer measurements, the resulting performance indicators of heat exchangers do not result in a minimum uncertainty possible. Also, this approach fails to resolve discrepancies in resulting transport performance parameters depending on the use of UA-LMTD method or effectiveness-NTU method. In this paper, heat exchanger performance data with two heat transfer measurements from hot and cold fluid streams are combined to produce a least uncertainty of the performance indicators. Individual measurements of mass flow rates and temperatures are corrected by most likely errors based on their respective uncertainties. The validity of this method has been demonstrated by Monte-Carlo simulations. Using air conditioning heat exchanger performance data under dry and wet surface conditions, it is demonstrated that the proposed method leads to a minimum uncertainty of the calculated variables.

Adsorption isotherm and adsorption-capillary transition theories have been developed based on homogeneous micro-/nanoporous materials and structures. However, material and structures are often heterogeneous including local surface roughness and defects, where no predictive tool is available so far. In this study, the adsorption isotherm and the adsorption-capillary transition is examined for Ar-filled Pt nanogap ( L z = 5 nm) with nanoposts (one surface only) using Grand Canonical Monte Carlo (GCMC) simulations. Results show that the presence of the nanoposts causes a bimodal capillary transition and reduces the capillary transition pressure compared to the nanogap with both bare surfaces. The pressure difference between the bimodal transitions is pronounced with decreasing the nanopost pitch size. The larger nanopost height also leads to the early capillary transition, but the bimodal transition is pronounced for moderate heights of the nanoposts. A stronger solid-fluid interaction reduces the adsorption-capillary transition pressure at given temperature and increases the transition pressure difference between the nanogaps with or without nanoposts. The obtained results provide new insights of the role of surface nanostructure (nanoposts) into adsorption isotherm and capillary transition.

We show that coherent thermal emission from an amorphous SiO 2 thin film can be significantly enhanced by placing it on top of a photonic crystal (PC). To demonstrate this principle, we simulated the reflectance and transmittance of a 1 micron thick layer of SiO 2 on a 20 layers PC using the scattering matrix method and finite difference numerical computations. Emissivity, calculated using Kirchhoff’s law, reaches unity at a peak wavelength around 10 microns due to overlapping of the PC’s forbidden band with bulk phonon-polariton modes in SiO 2 . This region of the electromagnetic spectrum is of particular interest for many technological applications as it corresponds roughly to maximum thermal emission from a blackbody at room temperature. In order to validate these theoretical predictions, a PSi multilayer was fabricated by electrochemical dissolution of p-type Si in HF with current density modulated as a function of time to produce alternating layers of two different porosities with different refractive indices. A 1 micron thick layer of SiO 2 was then deposited on top of the resulting PC by plasma enhanced chemical vapor deposition (PECVD). Reflectance of the PC was measured at normal incidence using a Fourier Transform Infrared Spectrometer (FTIR) before and after SiO 2 deposition and matches the simulations.

A high-fidelity computational fluid dynamics (CFD)-based model for bubble nucleation of the refrigerant HFE7100 on micrometer-featured surfaces is presented in this work. The single-fluid, incompressible Navier-Stokes equations, along with energy transport and natural convection effects, are solved on a featured surface resolved grid. An a priori cavity detection method is employed to convert raw profilometer data of a surface into well-defined cavities. The cavity information and surface morphology are represented in the CFD model by geometric mesh deformations. Surface morphology is observed to initiate buoyancy-driven convection in the liquid phase, which in turn results in faster nucleation of cavities. Simulations pertaining to a generic rough surface show a trend where smaller size cavities nucleate with higher wall superheat. This local-scale model will serve as a self-consistent connection to larger device scale continuum models where local feature representation is not possible.

New measures of Digital Thermal Monitoring (DTM), a low cost non-invasive tool to assess vascular health, are developed. Original measures of thermal vascular reactivity tend to be dependent upon the initial vascular state (such as temperature before occlusion). The objective of this study is to identify new measures that will provide a single independent predictor of vascular health. Full three dimensional simulations were carried out to simulate transient finger tip temperature response during brachial occlusion and reperfusion. These numerical results were validated with patient data and then used to simulate a variety of test cases and discern relationships between the thermal measure and the surge in flow rate (reactive hyperemia). Defining the zero-reactivity response as the response to a step change in flow rate from near zero back to the original steady state value before occlusion, a reference signal is constructed which accounts for the finger size, thermo-physical properties, external environment and the start temperature (initial vascular state). This reference is used to normalize the temperature reactivity, forming a new measure called adjusted temperature reactivity that is a function of reactive hyperemia only. It is independent of the initial vascular state and external environment, leading to a more robust and reliable measure of vascular health.

This study optimizes the performance of a mixing ventilation system coupled with a personalized ventilator that emits a cool sinusoidal horizontal airflow jet towards the occupant upper body in order to achieve good overall thermal comfort and good air quality in the occupant breathing zone. A transient 3-D computational fluid dynamics (CFD) model coupled with a transient bio-heat model was deployed to predict airflow and temperature fields in the space and around the occupant as well as segmental skin temperature profiles for local and overall thermal sensation and comfort analysis. Simulations were performed using the CFD model to determine the airflow optimal supply frequency, mean flow rate and amplitude at room temperature of 25 °C and PV jet temperature of 22 °C. The system also showed, that when increasing frequency at fixed mean flow rate, thermal comfort increased from by 15.2 %. However when increasing mean flow rate at a fixed frequency, thermal comfort dropped at the low frequency of 0.3 Hz but remained acceptable at the higher frequency of 0.5 Hz.

Experiments and simulations were performed to assess the performance of an HVAC system for the cooling of a Leitz Infinity coordinate measurement machine (CMM) enclosed within a glove box (GB). Manufacturer specifications require maintaining very uniform temperatures with spatial and temporal variations not to exceed 0.3 °C/hr, 0.4 °C/day, and 0.1°C/m. Data were collected at 0.17 Hz by 2 thermocouples located outside the glovebox, 10 static thermocouples located inside the glovebox, and up to 28 thermocouples attached to the moving granite table of the CMM. The latter thermocouples are arrayed in a grid in the volume of interest (VOI) which envelopes the motion of the CMM measuring head above the granite table. Data were collected for periods ranging from 1 to 5 days to observe the effects of temperature variations within the enclosing facility. Simulations were then performed on the enclosed volume of the GB using ANSYS-CFX to better understand the heat loads, and test temperature variation mitigation strategies. These simulations consisted of 18 runs which varied heat input from the CMM motors, inflow gas temperature from the HVAC system into the GB, and non-uniform GB wall temperature boundary conditions. Heat loads from the motors were found to be insignificant influences on the temperature distribution, while fluid entrainment inside the diffuser was discovered to lead to an adverse temperature distribution, and insufficient cooling in the VOI. Velocity distributions were examined by using a TSI VelociCalc 8345 to verify the presence of stagnant regions in the GB. Finally, modifications to the diffuser design were proposed to eliminate entrainment, improve the flow distribution, and enhance temperature uniformity.

In this work, an anti-icing “Piccolo tube” system is numerically analyzed, in order to investigate the efficiency of hot air impinging jets on internal wing surface. The model is two-dimensional and it is composed by the wing profile and an external volume. External air is considered a mixture, composed by dry air and water-vapor. A steady state problem with standard k-ε model is assumed. Numerical simulations are performed considering an average coefficient on the external surface and thermofluidynamic field inside the wing due to the anti-icing system. Numerical solutions are carried out by means of the Ansys-FLUENT code. Simulations are performed considering two values of the angle of attack, 0° and 4°, and two Mach number values, 0.576 and 0.700, related to the internal flow. esults are presented in terms of velocity and temperature fields, wall temperature profiles along the wing surfaces. As comparison, a case with only dry air in the external domain is performed. Results indicate that in presence of droplets the anti-icing system is less efficient than in the ideal case of dry air. Furthermore, a larger angle of attack improves the performance of the anti-icing system, but a larger Mach number deteriorates it.

Due to their relatively high capital and environmental cost of two-tank molten salt thermal storage systems, a significant amount of research has gone into looking for sensible and latent thermal energy storage alternatives suitable for concentrated solar thermal (CST) plants. Despite a large number of developments in the last decade, comparative studies among promising options have been lacking. In particular, only a few comparative studies are available in which thermal energy storage (TES) systems are integrated as an active subcomponent of CST plant. Therefore, this study compares selected sensible heat thermal energy storage systems based on their integrated performance with other CST components (e.g. a tower -based CST plant with a Rankine cycle) over a year of operation. In the present study, annual performances of single-medium thermocline (SMT), double-medium thermocline (DMT), and shell-and-tube (ST) system were compared with that of a conventional two-tank molten salt storage system. Concrete with porosity of 0.2 (concrete occupies 80% of the system) was selected as a low cost filler material in the DMT and ST systems. The systems were sized for 15 hours of storage capacity and integrated into a validated 19.9 MWe Gemasolar power plant model with solar multiple of 2.5. Before performing annual integrated simulations, an optimum design of each storage system was selected based on a performance analysis of the storage system over a constant 15 hours discharge. A CST plant with a two-tank molten salt system enables the highest amount of electricity generation in a year followed by the SMT and DMT systems, which resulted in 7% and 9% less electricity generation, respectively. For the CST plant with ST system, 20% less electricity was generated over a year. Overall, this study provides a methodology for the comparison of the TES alternatives, and it gives insight the most promising alternative for replacing two-tank molten salt systems.

This study presents a transient three-dimensional numerical study on fluid flow and heat transfer of flat-tube array using large eddy simulation (LES) covering both laminar and turbulent flow regimes. The simulations were performed in a rectangular region containing only one tube with periodic conditions specified on all boundaries. A staggered flat-plate array was first studied, and an existing solution was used for validation purpose. The numerical models were then applied to an in-line array composed of flat tubes with an aspect ratio of 0.25 and fixed tube spacings. By varying the in-flow velocity, the tube array was studied over a wide range of Reynolds number (600–12000). Temperature, velocity, and turbulent kinetic energy distributions as well as the interactions between them are presented and analyzed. Furthermore, the local heat transfer rate was analyzed along the various parts of the tube (leading edge, flat-top and wake or trailing-edge regions). Heat transfer correlation for each region of the tube and the entire tube array is proposed.

Engineering courses such as thermodynamics, fluid mechanics and heat transfer always involve many abstract math, physics concepts and equations — which are difficult to teach and understand. As fundamental courses in engineering programs, they are sometimes taught in big class size — where students may not receive adequate attention and assistance from instructors. To improve the teaching and learning efficiency, we proposed to develop virtual reality based interactive modules for learning computational fluid dynamics. In this paper, case-study learning module is demonstrated for conduction heat transfer. The programming languages of C# and Unity3D were used for the software development. Computational fluid dynamics simulation results obtained from ANSYS/FLUENT were incorporated in the program. The program has the integrated modules of mobility, interactivity, and controllability for the 3D modeling and simulations. Each module was developed separately for facilitating the program management, extension, and upgrades in the future. The developed interactive programs, incorporating rich, interactive, and engaging learning contexts, will help students gain and apply knowledge to solve real-world problems in mechanical engineering.

The effects of droplet train impingement on spreading-splashing transition and surface heat transfer were investigated experimentally and numerically. Experimentally, a single stream of HFE-7100 droplet train was generated using a piezo-electric droplet generator with the ability to adjust parameters such as droplet impingement frequency, droplet diameter and droplet impingement velocity. A thin layer of Indium Tin Oxide (ITO) was coated on a translucent sapphire substrate, which was used as heating element. High-speed and infrared imaging techniques were employed to characterize the hydrodynamics and heat transfer of droplet train impingement. Numerically, the high frequency droplet train impingement process was simulated using ANSYS-Fluent with the Volume of Fluid (VOF) method [1]. The heat transfer process was simulated by applying constant heat flux conditions on the droplet receiving surface. Droplet-induced spreading-splashing transition behavior was investigated by increasing the droplet Weber number while holding flow rate constant. High speed crown propagation images showed that at low-Weber number ( We < 400), droplet impingements resulted in smooth spreading of the droplet-induced crown. However, within the transitional droplet Weber number range ( We = 400–500), fingering and splashing (i.e. emergence of secondary droplets) could be observed at the crown’s rim. At high droplet Weber number ( We > 800), breakup of the crown was observed during the crown propagation process in which the liquid film behaved chaotically. Droplet-induced spreading-splashing transition phenomena were also investigated numerically. Reasonable agreement was reached between the experimental and numerical results in terms of crown morphology at different droplet Weber number values. The effects of spreading-splashing transition on surface heat transfer were also investigated at fixed flow rate conditions. Time-averaged Infrared (IR) temperature measurements indicate that heat flux-surface temperature curves are linear at low surface temperatures and before the onset of dry-out, which indicate that single phase forced convection is the primary heat transfer mechanism under those conditions. Numerical heat transfer simulations were performed within the single phase forced convection regime only. Instantaneous numerical results reveal that droplet-induced crown propagation effectively convect heat radially outward within the droplet impingement zone. Under high heat flux conditions, a sharp increase in surface temperature was observed experimentally when dry-out appeared on the heater surface. It was also found that strong splashing ( We > 800) is unfavorable for heat transfer at high surface temperature due to the onset of instabilities seen in the liquid film, which leads to dry-out conditions. In summary, the results indicate that droplet Weber number is a significant factor in the spreading-splashing transition and surface heat transfer.

The focus of this research is to predict the flow and heat transfer in a rotating two-pass duct geometry with staggered ribs using Large-Eddy Simulations (LES). The geometry consists of a U-Bend with 17 ribs in each pass. The ribs are staggered with an e/D h = 0.1 and P/e = 10. LES is performed at a Reynolds number of 100,000, a rotation number of 0.2 and buoyancy parameters (Bo) of 0.5 and 1.0. The effects of Coriolis forces and centrifugal buoyancy are isolated and studied individually. In all cases it is found that increasing Bo from 0.5 to 1.0 at Ro = 0.2 has little impact on heat transfer. It is found that in the first pass, the heat transfer is quite receptive to Coriolis forces which augment and attenuate heat transfer at the trailing and leading walls, respectively. Centrifugal buoyancy, on the other hand has a bigger effect in augmenting heat transfer at the trailing wall than in attenuating heat transfer at the leading wall. On contrary, it aids heat transfer in the second half of the first pass at the leading wall by energizing the flow near the wall. The heat transfer in the second pass is dominated by the highly turbulent flow exiting the bend. Coriolis forces have no impact on the augmentation of heat transfer on the leading wall till the second half of the passage whereas it attenuates heat transfer at the trailing wall as soon as the flow exits the bend. Contrary to phenomenological arguments, inclusion of centrifugal buoyancy augments heat transfer over Coriolis forces alone on both the leading and trailing walls of the second pass.

Jet impingement flow is known to generate one of the highest single-phase heat transfer rates, with potential for micro-electronics cooling applications. Although free-surface jets have been studied extensively, existing models are either too complex for practical use or do not consider all relevant parameters, such as the impinging jet’s velocity profile. Recently the authors have shown that the stagnation zone heat transfer is dictated by the jet’s centerline velocity upon impingement, and that going between the limiting cases (uniform vs. parabolic profiles, laminar flow) corresponds to a two-fold increase in heat transfer. In the present study, which is motivated by cooling at micro-scales (predominantly laminar flows), this simplified analysis is extended leading to a first-order analytical approximation, which is valid not only for the limiting cases but over the entire profile range. Thereby, the development of the jet flow both in the nozzle (pipe-type) and subsequently during its flight (before impingement) is incorporated in this model over a broad range of parameters. For validation of the model, as well as for additional insight into the governing physics, direct numerical simulations were conducted. Through which it is shown that the jet’s velocity profile and its evolution during free “flight” are dependent on the level of the flow’s upstream development in the nozzle, both of which depend on a single self-similar scale: distance travelled normalized by the nozzle diameter and Reynolds number. This one-way coupling requires incorporation of both stages of development for an accurate description, as done in the present model. During pipe-flow, the first-order model employs a more-rapid development rate than during jet-flight (due to the additional pressure-driven flow) — converging to more complex, well-known models, within a few pipe diameters (for Re = 200 to 2300). During flight, the model describes velocity profile relaxation, which is dominated by viscous diffusion and assisted by jet contraction. Jet contraction is dependent on the emerging velocity profile and liquid-vapor surface tension. For most relevant conditions surface tension is negligible, under which the first-order model centerline velocity decay prediction agrees well with both present simulations and previous works. Thereby, the present work lays the foundation for a simpler, more useable model for predicting heat transfer under an impinging free-surface jet, over a wide range of conditions (various liquids, pipe-type nozzles of different lengths, flow-rates and nozzle-to-plate distances), as part of an ongoing study into micro-jet array heat transfer.

Plate-fin heat exchangers are widely used in industries especially aerospace, cryogenics, food and chemical process industries where high heat flux surface area per unit volume is of prime importance. These heat exchangers consists of series of corrugated plates (herringbone or chevron), separated by gasket sealing. Chevron angled plates are one of the most commonly used type of geometry. The complex design of chevron plate heat exchanger, induces high turbulence and flow reversals causing high heat transfer through the plates. This paper discusses about the computational fluid dynamics simulations conducted over a simplified geometry of Chevron Plate Heat Exchanger to understand the formulation of vortices at different Reynold’s number for various aspect ratios. A single phase laminar flow with periodic boundary condition is used for analysis of the fluid behavior in a unit pattern of the corrugation geometry. Based on different flow and geometric conditions, varying amounts of swirl-flows are observed and different behavior of shear stress and heat transfer plot along the length of the plate is observed. At higher Reynolds numbers (Re) , the re-circulations and mixing by the induced vortices causes significant rise of heat flux, with marginal increase in friction factor.

The current study uses phonon wave-packet simulations and calculates the phonon transmission rate to explore the contributions of the mass and the bond energy differences on the thermal boundary conductance at the interface between two dissimilar materials. The impact of interdiffusion and interface bond strength on the thermal boundary conductance are also studied. Results show that the difference in mass and bond energy of materials results in a difference in phonon dispersion relations. Thus the frequency dependence of phonon transmission rate is observed at the interface. The interdiffusion allows high frequency phonons to contribute to phonon energy transport by inelastically scattering into multiple lower frequency phonons. Therefore the different energy distribution at the interface is observed for different wavevectors when there is interdiffusion between two materials which results in increased strain at the interface. It is also found that applying different bond strengths has little effect on thermal boundary conductance at the interface unless this interface bond strength deviates significantly from the commonly used mixing rules.

Field plated GaN high electron mobility transistors (HEMTs) are widely preferred amongst other GaN HEMT devices because of their ability to regulate electric field at high power densities. When operated at high power densities, GaN HEMTs suffer significantly from the concentrated heating effects in a small region called hotspot located closer to the drain edge of the gate. Although; the stabilizing effect of field plate on the electrical field distribution in HEMTs is known by researchers, its effect on temperature distribution and the hotspot temperature is still not studied to a greater extend. For this purpose, finite element thermal modelling of devices with different sizes of field plates are performed using the joule heating distribution data obtained from 2D electrical simulations. Results obtained from such combined model show that the existence of a field plate changes the electrical field, therefore the heat generation distribution within device. Moreover; increasing the size of the field plate has an effect on the maximum temperature at the hotspot region. The results are used to analyze these effects and improve usage of field plates for high electron mobility transistors to obtain better temperature profiles.

Nanofluids — colloidal suspensions of nanoparticles in base fluids — are known to possess superior thermal properties compared to the base fluids. Various theoretical models have been suggested to explain the often anomalous enhancement of these properties. Liquid layering around the nanoparticle is one of such reasons. The effect of the particle size on the extent of liquid layering around the nanoparticle has been investigated in the present study. Classical molecular dynamics simulations have been performed in the investigation, considering the case of a copper nanoparticle suspended in liquid argon. The results show a strong dependence of thickness of the liquid layer on the particle size, below a particle diameter of 4nm. To establish the role of liquid layering in the enhancement of thermal conductivity, simulations have been performed at constant volume fraction for different particle sizes using Green Kubo formalism. The thermal conductivity results show 100% enhancement at 3.34% volume fraction for particle size of 2nm. The results establish the dominant role played by liquid layering in the enhanced thermal conductivity of nanofluids at the low particle sizes used. Contrary to the previous findings, the molecular dynamics simulations also predict a strong dependence of the liquid layer thickness on the particle size in the case of small particles.

AlGaN/GaN based high electron mobility transistors (HEMTs) have been intensively used due to their high-efficiency power switching and large current handling capabilities. However, the high power densities and localized heating in these devices form small, high temperature regions called hotspots. Analysis of heat removal from hotspots and temperature control of the entire device is necessary for the reliable design of HEMT devices. For accurate analysis of heat transfer using thermal simulations in such devices with heat transfer occurring at different length scales, a roadmap is needed. For this purpose relative importance of different heat transfer modes in removing heat from devices with different substrate materials, operating at different power densities while different boundary conditions are analyzed using two and three-dimensional COMSOL Multiphysics simulations. Results give the relative importance of different parameters on the heat removal mechanism from devices and provide a roadmap for building simpler yet still accurate thermal models for AlGaN/GaN HEMTs and similar devices.

Two-phase (non-boiling) flows have been shown to increase heat transfer in channel flows as compared with single-phase flows. The present work explores the effects of gas phase distribution such as volume fraction and bubble size on the heat transfer in upward vertical channel flows. A two-dimensional (2D) channel flow of 10 cm wide by 100 cm high is studied numerically. Numerical simulations are performed using the commercial computational fluid dynamics (CFD) code ANSYS FLUENT. The bubble size is characterized by the Eötvös number. The volume fraction and the Eötvö number are varied parametrically to investigate their effects on Nusselt number of the two-phase flows. All simulations are compared with a single-phase flow condition.