Experimental Measurements of the Flow and Heat Transfer of a Square Jet Impinging on an Array of Square Pin Fins

An experimental investigation was conducted to explore the flow behavior, pressure drop, and heat transfer due to free air jet impingement on square in-line pin fin heat sinks (PFHS) mounted on a plane horizontal surface. A parametrically consistent set of aluminum heat sinks with fixed base dimension of 25 × 25 mm was used, with pin heights varying between 12.5 mm and 22.5 mm, and fin thickness between 1.5 mm and 2.5 mm. A 6:1 contracting nozzle having a square outlet cross sectional area of 25 × 25 mm was used to blow air at ambient temperature on the top of the heat sinks with velocities varying from 2 to 20 m/s. The ratio of the gap between the jet exit and the pin tips to the pin height, the so-called tip clearance ratio, was varied from 0 (no tip clearance) to 1. The stagnation pressure recovered at the center of the heat sink was higher for tall pins than short pins. The pressure loss coefficient showed a little dependence on Re, increased with increasing pin density, and pin diameter, and decreased with increasing pin height and clearance ratio. The overall base-to-ambient thermal resistance decreased with increasing Re number, pin density and pin diameter. Surprisingly, the dependence of the thermal resistance on the pin height and clearance ratio was shown to be mild at low Re, and to vanish at high Re number.

Transient Heat Transfer in Microscale Porous Materials Heated by Microsecond Laser Pulse of High Power Density

A novel experimental method was developed to measure the rapid transient temperature variations (heating rate > 107 K/s) of porous samples heated by high surface heat fluxes. With a thin film (0.1 μm thick) resistance thermometer of platinum as the temperature sensor and a super-high speed digital oscilloscope (up to 100 MHz) as the data recorder, rapid transient temperature variation in a porous material heated by a microsecond laser pulse of high power density is measured. Experimental results indicate that for high heat transfer cases (q′ > 109 W/m2) with short durations (5 – 20 μs) of heating, non-Fourier heat conduction behaviors appear. The non-Fourier hyperbolic heat conduction model and the traditional Fourier parabolic model are employed to simulate this thermal case respectively and the FDM is used to perform the numerical analysis. The hyperbolic model predicts thermal wave behavior in qualitative agreement with the experimental data.

A Method of Power Generation From Low Temperature Medium

For a long time, how to utilize waste heat to generate electricity has been an interesting and challenging field for energy scientists. This paper presents a new method, ferrofluid power generator (FPG), which takes advantage of waste heat or solar energy to generate electricity in a multiple heating and cooling tube with alternate ferrofluid slug and vapor bubble structures. Based on this method, a new device, a thermomagnetic engine (TME) composed of a straight vacuum tube, a current induced coil, and magnet & ferrofluid slugs (MFS), was designed. Experimental results show that the expanding vapor bubbles push MFSs to generate drastic and continuous oscillating movements under the effect of heat. The pulse voltage signals from the induced coil demonstrate that the TME has a practical structure, potentially higher power generating capacity, and a stable operation.

Free Convection Thermal Interaction Between 2D Components Mounted on a Vertically Oriented PCB

In this paper, measurements are presented of the temperature and velocity fields about two PCBs, with an array of five equally spaced two dimensional ribs. The ribs are two dimensional approximations of the Super Ball Grid Array (SuperBGA) package from Amkor electronics. The temperature and Nusselt number distributions are measured using Digital Moire´ Subtraction Interferometry and PIV is used to measure the velocity field. The effect of substrate conductivity is examined, and the level of thermal interaction is quantified. It is found that substrate conductivity significantly alters the induced boundary layer flow and also the recirculating vortex structure external to it. It is also found that there is a trade-off between a downstream component being heated by the thermal energy of the plume from a lower component, and cooled by the kinetic energy of that plume. The spacing to length ratio, above which the cooling effect is greater, is three for components mounted on a board with a high effective conductivity (15 W/m K). The ratio is greater than three for PCBs with lower effective conductivities. Previous work in the literature indicates a ratio greater than four for components mounted flush with an adiabatic substrate.

Multi-Phase Flow and Condensation in Proton Exchange Membrane Fuel Cells

The porous electrodes in a proton exchange membrane fuel cell are characterized by multi-phase flow, involving liquid water and multispecies gases, that are undergoing both condensation and catalyzed reactions. Careful management of liquid water and heat in the fuel cell system is essential for optimizing performance. The primary focus of this study is thus on condensation and water transport, neither of which have yet been studied in as much detail as other aspects of fuel cell dynamics. We develop a two-dimensional model for multi-phase flow in a porous medium that captures the fundamental transport processes going on in the electrodes. The governing equations are discretized using a finite volume approach, and numerical simulations are performed in order to determine the effect of changing operating conditions on fuel cell performance.

Numerical Analysis of Heated Soil Vapor Extraction System

An innovative and highly effective technique for remediation of soil has been developed—Heated Soil Vapor Extraction (HSVE), which is one of essential technologies that quickly and effectively remediates soil that is contaminated with organic compounds. The system efficiently uses the principles of heat transfer and diffusion to eliminate organic compounds from the soil. It basically consists of a high temperature heat source and a sink to take away the vaporized compounds in the presence of high temperature in the soil. A numerical study has been conducted to further strengthen the fact that the system is very effective, by actually modeling soil with system. Finite Element Analysis software ANSYS® has been used for the purpose of analysis. Such analysis will help environmental science and give new dimensions to soil remediation processes to clean soil off volatile organic compounds so that they can be carried out quickly, efficiently and economically.

Computation of the Conjugating Heat Transfer of Fuel and Oxidant Separated by a Heat-Generating Cell Tube in a Solid Oxide Fuel Cell

A numerical model is presented in this work to compute the inter-dependent fields of flow, temperature and the concentrations of multiple gases in a single tubular solid oxide fuel cell (SOFC) system. It was supposed that the fuel gas supplied to the fuel cell is from a pre-reformer and thus contains hydrogen and proportions of carbon monoxide, carbon dioxide, steam, and methane. The model takes mixture gas properties of the fuel and oxidant as functions of the numerically obtained local temperature, pressure and species concentrations, which are inter-dependent and intimately related to the electrochemical reaction in the SOFC. In the iterative computation steps, local electrochemical parameters were simultaneously calculated based on the local parameters of pressure, temperature, and concentration of the species available at each step. Upon the convergence of the computation, both local details and the overall performance of the fuel cell could be obtained. The numerical results obtained are helpful for better understanding of the operation of SOFCs.

Hybrid Numerical Method for Heat Transfer Analysis of Complex 3D Geometries

The heat transfer analysis of systems with complex 3D geometries is usually done by numerical methods. Finite Element Method (FEM) and Finite Difference Methods (FDM) are widely used for this purpose. Complex geometries are accurately analyzed by FEM method. However, FEM solutions can be computationally inefficient for thermal problems that have high mesh densities with complex boundary conditions and variable material properties. On the other hand, Finite Difference method (FDM) is difficult to apply for complex geometric shapes. A hybrid numerical approach that combines the advantages of FDM and FEM has been integrated into a thermal simulation code. The hybrid technique has been implemented using object oriented programming techniques in a PC environment. A comparison of the computational efficiency of the two methods has been presented.

Simulation of Single-Event Failure in Power Diodes

Single-event burnout (SEB) is a catastrophic failure mechanism in power diodes that is initiated by the passage of a heavy ion through a diode in a current-blocking state. In this work, the physical mechanism responsible for device failure during SEB is investigated using transient, coupled electro-thermal, device simulations. For the first time, the effects of a thermal feedback mechanism have been examined and deemed crucial to predicting possible failure in power diodes. Results indicate that device failure is predicted for large blocking voltage near breakdown with a linear energy transfer (LET) of 30MeV/mg/cm2 only when thermal effects are included. However, without inclusion of the thermal model, no device failure is predicted. These results correspond to experimental observations better than any previous work.

Rheological Study of Micro Particle Laden Polymeric Thermal Interface Materials: Part 1 — Experimental

Particle laden polymers are one of the most prominent thermal interface materials (TIM) used in electronics cooling. Most of the research groups have primarily dealt with the understanding of the thermal conductivity of these types of TIMs. Thermal resistance is not only dependent on the thermal conductivity but also on the bond line thickness (BLT) of these TIMs. It is not clear that which material property(s) of these particle laden TIMs affects the BLT. This paper discusses the experimental measurement of rheological parameters such as non-Newtonian strain rate dependent viscosity and yield stress for 3 different particle volume fraction and 3 different base polymer viscosity materials. These rheological and BLT measurements vs. pressure will be used to model the BLT of particle-laden systems for factors such as volume fraction.