Abdlmonem H. Beitelmal

Effects of surface roughness on the average heat transfer of an impinging air jet

Abstract The effect of surface roughness of a uniformly heated plate on the average heat transfer characteristics of an impinging air jet was experimentally investigated. Two aluminum plates, one with a flat surface and the second with some roughness added to the surface were fabricated. The roughness took the shape of a circular array of protrusions of 0.5 mm base and 0.5mm height. A circular Kapton heater of the same diameter as the plates (70mm) supplied the necessary power. The surfaces of the plates were polished to reduce radiation heat losses and the back and sides insulated to reduce conduction heat losses. Temperatures were measured over a Reynolds number ranging from 9600 to 38500 based on flow rate through a 6.85mm diameter nozzle. The temperature measurements were repeated for nozzle exit-to-plate spacing, z/d, ranging from 1 to 10. The average Nusselt number for both cases was plotted versus the Reynolds number and their functional correlation was determined. The results indicate an increase of up to 6.0% of the average Nusselt number due to surface roughness. This modest increase provides evidence to encourage further investigation and characterization of the surface roughness as a parameter for enhancing heat transfer.

Introducing Energy Efficiency Metrics for Server and Data Centers

New servers and data center metrics are introduced to facilitate proper evaluation of data centers power and cooling efficiency. These metrics will be used to help reduce the cost of operation and to provision data centers cooling resources. The most relevant variables for these metrics are identified and they are: the total facility power, the servers’ idle power, the average servers’ utilization, the cooling resources power and the total IT equipment power. These metrics can be used to characterize and classify servers and data centers performance and energy efficiency regardless of their size and location.Copyright

Turbulent heat transfer study of inclined impinging jets

*† ‡ The heat transfer coefficient of impinging jet is several times higher than that of cross flow conditions, which makes it an attractive cooling solution for high power electronics enclosure or space constrained system. A number of parameters affect jet impingement heat transfer such as nozzle exit-to-target spacing, inclination angle, and Reynolds number. Numerical simulations for a two-dimensional air jet have been created using six well-known turbulent models. The objective of this exercise is to assess the capability of these models as it relates to jet impingement heat transfer. Four different cases were created and compared to the experimental results of Beitelmal et al. (2000). The deviation of the numerical results from the experimental ones varied between 1% to 50% with an average deviation of about 24%. None of the turbulence models used in this study showed any superior capabilities when dealing with jet impingement heat transfer application. However the current results showed that both one-equation Spalart-Allmaras and the RNG k-e models gave the best estimates for normally impinging jet with a percent deviation ranging between 3% and 32% and average deviation of 22% from the experimental results. The SST k-ω model results gave a better approximation in case of inclined jet heat transfer with an average deviation of 18% from the experimental results.

Energy performance investigation of a district cooling system

A steady-state model is developed to investigate the effect of the chilled water supply temperature and the inlet condenser water temperature on the performance of the centralized chiller system. The current results show that increasing the chilled water supply temperature by 1°C increases the coefficient of performance (COP) of the chiller by 1% to 3% and reduces the total cooling system power consumption by an average of 2% when operating the chiller system at the design capacity level. Decreasing the entering condenser water temperature by 1°C increases the chiller COP by an average of 2% and reduces the total power demand by an average of 1.5%. The reduction in the power consumption translates into a measurable annual reduction in CO2 emissions. The amount of CO2 emissions reduction depends on the type of fuel used to produce the electricity available for the chiller system. The current results also show that the chilled water temperature can be safely raised to a higher set point temperature than the industry standard set point of 5–6°C while preserving the cooling capacity requirements. In addition, the current results suggest that chilled water supply temperature set point of 8°C to 10°C (46°F to 50°F) would provide energy savings of 5% and 9.8%, respectively, over the base case used of 6°C while reducing the carbon footprint by the same percentage.

Numerical Investigation of Data Center Raised-Floor Plenum

Data center raised-floor plenum effectiveness is numerically investigated using a computational fluid dynamics (CFD) package to determine the most appropriate data center raised floor plenum height (size). The current study considers raised floor plenum height between 30.5 cm and 152.4 cm (12–60 inch) with the standard 15.2 cm (6-inch) increment while maintaining the supply airflow rate constant. Three factors are considered for optimum plenum size: the individual airflow rate from each perforated tile, the level of airflow rates uniformity between different perforated tiles and the top rack inlet air temperature at 2.13 m (7-ft) above the raised floor. The results show that raised floor plenum with height of 76.2 cm (30-inch) or higher had the most uniform airflow rates through the perforated tiles and most uniform temperature across the top of the racks. The findings also indicate that the uniformity of the static pressure inside the plenum increases with plenum height. The current results show that data center plenums with height range of 76.2–91.4 cm (30–36 inch) are found to be the best option based on the current thermo-fluid considerations however thermo-economics analysis should be addressed in future work.Copyright

Formula Electric System: Thermal Management Design

This paper presents the thermal management analysis performed on lithium polymer cells designed for High Performance Electric Vehicle (HPEV) applications. The objective was to choose an optimum temperature range for the cells to operate at, determine the thermal response of the cells under their full spectrum of discharge capabilities, calculate the necessary convective heat transfer necessary to maintain the cells within said temperature range, then to create a thermal management solution to incorporate into a battery pack composed of 288 cells. Thermal testing and modeling on individual lithium polymer cells determined the thermal response and amount of convection cooling required for the cells over their intended duty cycles. A convective heat transfer coefficient of 50 W/m2K was determined to be sufficient to prevent the proposed cell from exceeding the optimum temperature range during its most strenuous duty cycle. The proposed design scheme utilized a fan to force air circulation up along the side of modules where each module consists of four cells connected in series. A proposed feedback control loop system allowed for active control of the battery cell’s temperature resulting in an increase in efficiency and overall performance for HPEV applications.Copyright

Introducing a Novel Reactor Concept: Indirectly Fired Integrated Gasification and Steam Generation System

This paper introduces a novel gasification reactor that uses steam gasification of carbonaceous feedstock by indirectly heating the reacting flow through a high temperature heat exchanger without the need for partial combustion with oxygen. It demonstrates the importance of gasification as a method for increasing power plant efficiency and reducing emissions. This paper also describes the computational model created to model this novel gasifier and the results of the model that illustrates the efficiency and purity advantages of the new gasifier. The reactor was modeled as a 1D counter-reacting flows heat exchanger, using the effectiveness-number of transfer units (e-Ntu ) method. The heating flow was assumed to be fully combusted at the inlet. The gasification stream was modeled as a plug flow, where the reaction is kinetically controlled. A simplified version of the Random Pore Model (RPM) was used to predict the char consumption. The results indicate that the gasification of coal with steam without partial combustion with oxygen using this new concept is feasible. The gasification reaction rates are found to be slow at temperatures less than 1200°C, but most of the char conversion, which reached about almost 100% completion, occurred at higher than 1200°C.Copyright