Analysis and Mitigation of Sample Heating in Thermoreflectance Microscopy

The illumination of a sample when imaged by thermoreflectance thermal microscopy may cause significant heating of the surface. Nonlinearities in the performance of the system being imaged may lead to large measurement induced errors in the observed temperature field. Analytical expressions are presented to estimate the temperature rise and heat flux in a sample. Spatially filtered thermo-reflectance microscopy is introduced as a technique to significantly reduce the incident heat flux without loss of spatial resolution. This paper was also originally published as part of the Proceedings of the ASME 2005 Pacific Rim Technical Conference and Exhibition on Integration and Packaging of MEMS, NEMS, and Electronic Systems.

Remote Heat Pipe Based Heat Exchanger Performance in Notebook Cooling

Remote heat pipe based heat exchanger cooling systems are becoming increasingly popular in cooling of notebook computers. In such cooling systems, one or more heat pipes transfer the heat from the more populated area to a location with sufficient space allowing the use of a heat exchanger for removal of the heat from the system. In analsysis of such systems, the temperature drop in the condenser section of the heat pipe is assumed negligible due to the nature of the condensation process. However, in testing of various systems, non linear longitudinal temperature drops in the heat pipe in the range of 2 to 15 °C, for different processor power and heat exchanger airflow, have been measured. Such temperature drops could cause higher condenser thermal resistance and result in lower overall heat exchanger performance. In fact the application of the conventional method of estimating the thermal performance, which does not consider such a nonlinear temperature variations, results in inaccurate design of the cooling system and requires unnecessarily higher safety factors to compensate for this inaccuracy. To address the problem, this paper offers a new analytical approach for modeling the heat pipe based heat exchanger performance under various operating conditions. The method can be used with any arbitrary condenser temperature variations. The results of the model show significant increase in heat exchanger thermal resistance when considering a non linear condenser temperature drop. The experimental data also verifies the result of the model with sufficient accuracy and therefore validates the application of this model in estimating the performance of these systems. This paper was also originally published as part of the Proceedings of the ASME 2005 Pacific Rim Technical Conference and Exhibition on Integration and Packaging of MEMS, NEMS, and Electronic Systems.

Spiral-Plate Heat Exchanger Effectiveness With Equal Capacitance Rates

A formula is derived for the dependence of heat exchanger effectiveness on the number of transfer units for a spiral-plate heat exchanger with equal capacitance rates. The difference-differential equations that describe the temperature distributions of the two counter-flowing fluids, neglecting thermal radiation, are solved symbolically to close approximation. Provision is made for offset inlet and exit of the hot and cold fluids at the outer periphery and for large heat transfer coefficients in entrance regions. The peak effectiveness and the number of transfer units at which it occurs are predicted.

Study on the Heat Transfer in the Water Pool Type Reactor Cavity Cooling System of the Very High Temperature Gas Cooled Reactor

In the HTGR (High Temperature Gas Cooled Reactor), the Reactor Cavity Cooling System (RCCS) is equipped to remove the heat transferred from the reactor vessel to the structure of the containment. The function of the RCCS is to dissipate the heat from the reactor cavity during normal operation including shutdown. The system also removes the decay heat during the loss of forced convection (LOFC) accident. A new concept of the water pool type RCCS was proposed at Seoul National University. The system mainly consists of two parts, water pool located between the containment and reactor vessel and five trains of air cooling system installed in the water pool. In normal operations, the heat loss from the reactor vessel is transferred into the water pool via cavity and it is removed by the forced convection of air flowing through the cooling pipes. During the LOFC accident, the after heat is passively removed by the water tank without the forced convection of air and the RCCS water pool is designed to provide sufficient passive cooling capacity of the after heat removal for three days. In the present study, experiments and numerical calculations using CFX5.7 for the water pool and cooling pipe were performed to investigate the heat transfer characteristics and evaluate the heat transfer coefficient model of the MARS-GCR (Multi-dimensional Analysis of Reactor Safety for Gas Cooled Reactor Analysis) which was developed for the safety analysis of the gas cooled reactor. From the results of the experiments and CFX calculations, heat transfer coefficients inside the cooling pipe were calculated and those were used for the assessment for the heat transfer coefficient model of the MARS-GCR.

Numerical Optimization of the Thermoelectric Cooling Devices

The present study proposes the unified numerical approach to the problem of optimum design of the thermoelectric devices for cooling electronic components. The method is illustrated with several examples which are based on the standard mathematical model of a single-stage thermoelectric cooler with constant material properties. The model takes into account the thermal resistances from the hot and cold sides of the TEC. Values of the main physical parameters governing the TEC performance (Zeebeck coefficient, electrical resistance and thermal conductance) are derived from the manufacturer catalog data on the maximum achievable temperature difference, and the corresponding electric current and voltage. The independent variables for the optimization search are the number of the thermoelectric coolers, the electric current and the cold side temperature of the TEC. The additional independent variables in other cases are the number of thermoelectric couples and the height-to area ratio of the thermoelectric pellet. The objective for the optimization search is the maximum of the total cooling rate or maximum of COP. In the present study, the problems of optimum design of thermoelectric cooling devices are solved using the so-called Multistart Adaptive Random Search (MARS) method [16].

Evaporation and Nucleate Boiling of an Individual Droplet on Surfaces

A visual study was conducted to investigate the evaporation and nucleate boiling of a water droplet on heated copper, aluminum, or stainless surfaces with temperature ranging from 50°C to 112°C. Using a high-speed video imaging system, the dynamical process of the evaporation of a droplet was recoded to measure the transient variation of its diameter, height, and contact angle. When the contact temperature was lower than the saturation temperature, the evaporation was in film evaporation regime, and the evaporation could be divided into two stages. When the surface temperature was higher than the saturation temperature, the nucleate boiling was observed. The dynamical behavior of nucleation, bubble dynamics droplet were detail observed and discussed. The linear relationships of the average heat flux vs. temperature of the heated surfaces were found to hold for both the film evaporation regime and nucleate boiling regime. The different slopes indicated their heat transfer mechanism was distinct, the heat flux decreased in the nucleate boiling regime more rapidly than in the film evaporation due to the strong interaction between the bubbles.

Design and Calibration of a Novel High Temperature Heat Flux Gage

A new type of heat flux sensor (HTHFS) has been designed and constructed for applications at high temperature and high heat flux. It is constructed by connecting solid metal plates to form brass/steel thermocouple junctions in a series circuit. The thermal resistance layer of the HTHFS consists of the thermocouple materials themselves, thus improving temperature limits and lowering the temperature disruption of the sensor. The sensor can even withstand considerable erosion of the surface with little effect on the operation. A new type of convection calibration apparatus was designed and built specifically to supply a large convection heat flux. The heat flux was supplied simultaneously to both a test and standard gage by using two heated jets of air that impinged perpendicularly on the surface of each gage. The sensitivity for the HTHFS was measured to have an average value of 20 μV/(W/cm2). The uncertainty in this result was determined to be ±10% over the entire range tested. The sensitivity agrees with the theoretically calculated sensitivity for the materials and geometry used. Recommendations for future improvements in the construction and use of the sensors are discussed.

The Effect of Reynolds Number on the Aerodynamic Performance of Micro Radial Flow Fans

Elevated heat dissipation and simultaneous reductions in package sizes are well documented for a range of electronics systems. The problem is heightened in portable systems where the space available for the implementation of an active cooling methodology is limited and conventional cooling products are too large. Using micro scale radial flow fans is a potential solution. However, little is known about the aerodynamic effects of reducing the fan scale and therefore Reynolds number to the extent required for typical portable electronic applications. This paper investigates this issue, by quantifying the reduction in aerodynamic performance which accompanies the reductions in scale. To do this, geometrically similar radial flow fans were fabricated with diameters ranging from 80 to 10mm. Measurements of the rotors’ geometries are presented, showing a high degree of geometric similarity between the fans. The aerodynamic performance of each of the fans was measured. Non-dimensional performance of each of the larger fans were almost identical, while the performance plot of the smallest fan differed significantly from the others. The paper tentatively concludes that a fundamental change in flow phenomena has emerged in the smallest scale fan which has altered its aerodynamic characteristics.

A Simulation of the Solid Oxide Fuel Cell Electrochemical Light Off Phenomenon

During latter-stage, “start-up” heating of a solid oxide fuel cell (SOFC) stack to a desired operating temperature, heat may be generated in an accelerating manner during the establishment of electrochemical reactions. This is because a temperature rise in the stack causes an acceleration of electrochemical transport given the typical Arrhenius nature of the electrolyte conductivity. Considering a potentiostatic condition (i.e., prescribed cell potential), symbiosis thus occurs because greater current prevalently leads to greater by-product heat generation, and vice versa. This interplay of the increasing heat generation and electrochemistry is termed “light off”, and an initial model has been developed to characterize this important thermal cycling phenomenon. The results of the simulation begin elucidating the prospect of using cell potential as well as other electrochemical operating conditions (e.g., reactants utilization) as dynamic controls in managing light off transients and possibly mitigating thermal cycling issues.

Thermal Transport Network Model for High-Porosity Materials: Application to Nanoporous Aerogels

Nanoporous thin films have received attention in the microelectronics field for their application as next-generation low-k inner-layer dielectric (ILD) materials due dielectric constants approaching 1.4. In addition, emerging applications as thermal insulation for microsystems aim to exploit the materials’ unique thermal properties in sensor and component products. However, its thermal properties can vary greatly depending on fabrication processes and material morphology. In addition, a variety of transport phenomena are present and delineation among them is difficult. In this work, we examine heat transport in aerogel, one of the most common embodiments of nanoporous materials, to identify the main modes of energy transport. We employ a modified diffusion-limited cluster aggregation (DLCA) technique to simulate aerogel’s highly porous, amorphous solid structure. Network models then simulate heat transport through the structure to extract effective thermal conductivity. The models are verified by comparing calculated bulk data to published aerogel literature. Preliminary models yield thermal conductivity on the order of 0.010 W/m*K, which is consistent with published data for aerogel films. These values vary inversely with porosity of the aerogel following an inverse power law often used to fit aerogel experimental data. This methodology is most useful for examining the sensitivity of thermal conductivity to salient structural features such as porosity, pore size distribution, solid thermal properties, average branch width, and sub-continuum phenomena. The results of this study can be used as a predictive tool in optimizing aerogel fabrication process to yield morphologies that best-suit the requirements of the application.