Jinny Rhee

Experimental estimate of the continuous one-dimensional kernel function in a rectangular duct with forced convection

The continuous, one-dimensional kernel function in a rectangular duct subject to forced convection with air was experimentally estimated using liquid crystal thermography techniques. Analytical relationships between the kernel function for internal flow and the temperature distribution resulting from a known heat flux distribution were manipulated to accomplish this objective. The kernel function in the hydrodynamically fully developed region was found to be proportional to the streamwise temperature gradient resulting from a constant heat flux surface. In the hydrodynamic entry region of the rectangular duct, a model for the kernel function was proposed and used in its experimental determination. The kernel functions obtained by the present work were shown to be capable of predicting the highly nonuniform surface temperature rise above the inlet temperature resulting from an arbitrary heat flux distribution to within the experimental uncertainty. This is better than the prediction obtained using the analytically derived kernel function for turbulent flow between parallel plates, and the prediction obtained using the conventional heat transfer coefficient for constant heat flux boundary conditions. The latter prediction fails to capture both the quantitative and qualitative nature of the problem. The results of this work are relevant to applications involving the thermal management of nonuniform temperature surfaces subject to internal convection with air, such as board-level electronics cooling. Reynolds numbers in the turbulent and transition range were examined.

The influence of personality and ability on undergraduate teamwork and team performance

The ability to work effectively on a team is highly valued by employers, and collaboration among students can lead to intrinsic motivation, increased persistence, and greater transferability of skills. Moreover, innovation often arises from multidisciplinary teamwork. The influence of personality and ability on undergraduate teamwork and performance is not comprehensively understood. An investigation was undertaken to explore correlations between team outcomes, personality measures and ability in an undergraduate population. Team outcomes included various self-, peer- and instructor ratings of skills, performance, and experience. Personality measures and ability involved the Five-Factor Model personality traits and GPA. Personality, GPA, and teamwork survey data, as well as instructor evaluations were collected from upper division team project courses in engineering, business, political science, and industrial design at a large public university. Characteristics of a multidisciplinary student team project were briefly examined. Personality, in terms of extraversion scores, was positively correlated with instructors’ assessment of team performance in terms of oral and written presentation scores, which is consistent with prior research. Other correlations to instructor-, students’ self- and peer-ratings were revealed and merit further study. The findings in this study can be used to understand important influences on successful teamwork, teamwork instruction and intervention and to understand the design of effective curricula in this area moving forward.

Performance-cost optimization of a diamond heat spreader

The steady-state thermal resistance of a small heat source applied to a diamond heat spreader attached to a larger substrate with Newtonian cooling on the opposite side was evaluated using numerical simulation. The substrate with Newtonian cooling models a range of convection-cooled designs, such as a heat sink base with a finned surface or the base of a cold plate with liquid cooling. The objective of this work is to quantify the thermal performance of the modeled system as a function of the diamond heat spreader size and properties. The resulting maximum thermal resistance as a function of diamond spreader thickness, lateral dimension, and conductivity are presented, as well as some guidelines for effective thermal design. In addition, a range of convection conditions typical for these applications are examined. Synthetic diamond is still an expensive material, ranging from

Thermal Spreading Resistance for Square and Rectangular Entities

1 to

Spatial and Temporal Resolution of Conjugate Conduction-Convection Thermal Resistance

20 per square millimeter in the lateral plane. The price increases sharply with conductivity, thickness, and lateral dimension, all of which increase thermal performance of the diamond spreader. For this reason, the cost per thermal performance is a distinctly different optimization problem from the thermal performance alone. The results of this optimization are presented for a Biot number typical for forced convection through a finned surface using direct air cooling.

Numerical Optimization of a Cogenerating Parabolic Solar Collector Receiver

A systematic study was performed using numerical modeling to understand the effects of thermal spreading resistance on the total resistance from junction to ambient for square and rectangular entities. Simultaneously, a literature survey was conducted to understand effects of thermal spreading resistance on circular, square and rectangular heat sources and heat spreading plates. Simulation from numerical model (Flotherm) was then compared with results from analytical solution derived by Lee and Ellison for square and rectangular heat sources respectively as a benchmarking process for the numerical model. The model presumes a heat source on a large cooling plate beneath which has a constant heat transfer coefficient on the sink side. Numerical simulations were performed for steady-state conditions with different range of dimensions for source to plate edge length ratio and dimensionless thickness. Results for dimensionless spreading resistance as a function of relative contact size, plate thickness and Biot number were derived using Flotherm and compared with the analytical models mentioned above. Results show that numerical modeling using software like Flotherm can offer excellent results within acceptable percentage difference when compared to analytical solutions

Characterization of airflow impedance in two types of telecommunications chassis

A transient, 3-D solution to the heat conduction equation with a small square heat source on an adiabatic surface and Newtonian convection on the opposite side was obtained using Greens functions. The geometry conservatively models conduction spreading resistance encountered by small, concentrated heat sources such as light-emitting diodes and integrated circuits in general, mounted to larger substrates such as the base of a heat sink experiencing Newtonian convection. The solution is presented for a range of nondimensional parameters. Superposition techniques can also be used to extend the applicability of the current solution to the temperature prediction of arbitrary heat flux patterns in certain cases. This technique only holds for applications where the heat transfer coefficient is not a function of temperature, such as thermal management strategies designed to rely on forced convection with air.

Supporting student career development of undergraduate engineering

The motivation for this study comes from the need for a clean, renewable energy source, which is greater now more than ever to reduce the country’s dependence on fossil fuels. Cogenerating solar systems can provide heat and electricity for many industrial applications such as power generation and absorption refrigeration systems. For example, data centers that run on conventional refrigeration systems are one of the largest electricity consumers in the nation, accounting for 1.2% of the total electricity consumption in 2005. This electricity consumption, almost half of which is used to run the data center’s air conditioning units, translates to

Domestic Hot Water Storage Tank: Design and Analysis for Improving Thermal Stratification

2.7 billion in electricity costs for that year. Using cogenerating solar systems for these types of applications could represent a significant amount of savings in electricity costs. The objective of this paper is to numerically optimize a receiver for a cogenerating photovoltaic and thermal parabolic solar collector that will produce both heat and electricity. The solar cogeneration system studied will convert solar energy into both heat and electricity by using a combination of photovoltaic cells, a parabolic trough thermal collector, and water as the liquid heat exchanger on the photovoltaic cells. The peak electrical efficiency of the multi-junction gallium arsenide Spectrolab photovoltaic cells used in this study is about 32%, with the rest of the solar energy being absorbed as heat. These temperature gains in the cells can lead to a decrease in efficiency. However, in cogenerating systems, water is used as a working fluid to remove heat from the photovoltaic cells, thus aiding in increasing the electrical efficiency of the photovoltaic system as well as increasing the thermal energy gained from the solar thermal collector. The numerical analysis for this project will use Flotherm, a CFD tool used to solve fluid and thermal problems. A single-phase water cooled square duct receiver subjected to non-uniform heating will be analyzed in Flotherm to determine the optimal parameters for the best convection heat transfer between the working fluid and the photovoltaic cells. To enhance the heat transfer between photovoltaic cells and working fluid, the inner surface of the receiver tube receiving the heat flux will be improved by adding fins to increase heat transfer and induce turbulent flow. The initial receiver design will be compared with other receivers to determine the optimal design. Results will be presented parametrically for a range of flow rates and receiver geometry.Copyright

Lead-Free, Fluxless Solder Joints to Synthetic Diamond

The airflow impedance curve for two common configurations used in telecommunications chassis were measured and examined in non-dimensional coordinates, such that data obtained at sea level may be scaled to increased altitudes. The results show that the non-dimensional airflow impedance curves for both chassis configurations predict minimal changes in air velocity with increased altitude for most of the Reynolds number ranges studied. However, for one chassis configuration studied, a slight decrease in air velocity is predicted with increased altitude. Because a nonconservative component temperature predictions at high altitude will result if constant air velocity is assumed, the decrease in air velocity should be accounted for if there are critical electronic components operating near their maximum temperatures. Reliable operation of electronic components in telecommunications equipment up to 4000 m (13,000 ft.) is a requirement of the Network Equipment and Building Standard (NEBS).