Thesis (MScEng (Mechanical and Mechatronic Engineering))--Stellenbosch University, 2008. / This study evaluates the performance characteristics of a power plant incorporating a steam turbine and a direct air-cooled dry/wet condenser operating at different ambient temperatures. The proposed cooling system uses existing A-frame air-cooled condenser (ACC) technology and through the introduction of a hybrid (dry/wet) dephiegmator achieves measurable enhancement in cooling performance when temperatures are high. In order to determine the thermal-flow performance characteristics of the wet section of the dephlegmator, tests are conducted on an evaporative cooler. From the experimental results, correlations for the water film heat transfer coefficient, air-water mass transfer coefficient and the air-side pressure drop over a deluged tube bundle are developed. During periods of high ambient temperatures the hybrid (dry/wet) condenser operating in a wet mode can achieve the same increased turbine performance as an oversized air-cooled condenser or an air-cooled condenser rith adiabatic cooling (spray cooling) of the inlet air at a considerably lower cost. For the same turbine power output the water consumed by an air-cooled condenser incorporating a hybrid (dry/wet) dephlegmator is at least 20% less than an air- cooled condenser with adiabatic cooling of the inlet air. / Sponsored by the Centre for Renewable and Sustainable Energy Studies, Stellenbosch University

Advancements in the networking and routing industry have created higher power electronic systems which dissipate large amounts of heat while cooling technology for these electronic systems has remained relatively unchanged. This report illustrates the development and testing of a hybrid liquid-air cooling system prototype implemented on Cisco’s 7609s router. Water was the working fluid through cold plates removing heat from line card components. The water was cooled by a compact liquid-air heat exchanger and circulated by two pumps. The testing results show that junction temperatures were maintained well below the 105°C limit for ambient conditions around 30°C at sea level. The estimated junction temperatures for Cisco’s standard ambient conditions of 50°C at 6,000 feet and 40°C at 10,000 feet were 104°C and 96°C respectively. Adjustments to the test data for Cisco’s two standard ambient conditions with expected device characteristics suggested the hybrid liquid-air cooling design could meet the projected heat load.

Reunir o condicionamento de ar indireto pela compressão a vapor, usando o fluido refrigerante R134a, ao princípio de resfriamento evaporativo direto, foi o tema foco do trabalho. O resfriamento a compressão de vapor tem como resultado, a entrega do ar condicionado sob condições de conforto que podem ser modificados de modo significativo, se for adicionado aos resultados, o condicionamento por resfriamento evaporativo. A observação dos experimentos se baseou nos processos psicrométricos envolvidos, aliando a saturação adiabática do ar no resfriamento evaporativo conjugado ao efeito frigorífico da diferença de entalpias na entrada e saída do evaporador no ciclo convencional de resfriamento. Foi possível avaliar a redução do coeficiente de desempenho e a capacidade de refrigeração aumentando em função do crescimento da rotação do compressor. No ensaio conjugado, resfriamento evaporativo e compressão a vapor instalados em série, observou-se a redução da efetividade do resfriador evaporativo, quando ocorre o aumento da vazão de ar, além do incremento da capacidade de refrigeração em função da associação dos dois sistemas, sem que houvesse aumento significativo da potência consumida. / Assemble the air conditioning by indirect vapor-compression, using refrigerant R134a, the principle of evaporative cooling direct cooling, was the subject focus of the work. The vapor compression cooling results in the delivery of air conditioning under conditions of comfort that can be changed significantly if we add the results, the conditioning by evaporative cooling. The observation of the experiments are bases on psychometric processes involved, combining the adiabatic saturation of air in the evaporative cooling effect of the combined refrigerator of the difference in enthalpy at the inlet and outlet of the evaporator in the conventional cooling cycle. It was possible to evaluate the reduction of the coefficient of performance and cooling capacity increase due to the increase of compressor speed. During the serial aligned pads evaporative cooling and vapor compression test, there was a decrease of the effectiveness evaporative cooler, when the air flow was added, in addition, it was increased the cooling capacity according to the association of the two systems, with no significant increase the power supplied.

An approach to hybrid cooling technique is proposed using air jets which impinge on a triangular grooved surface with dry grooves and grooves containing water. One major application is for condensers of thermoelectric power plants. The heat and mass transfer analogy were successfully used to evaluate the simultaneous heat and mass transfer. Results showed that hybrid jet impingement produced high heat flux levels at low jet velocities and flow rates. Experimental results were used to characterize the resulting heat transfer under different conditions such as flow open area percentage, array orifices diameter and array to surface stand-off distance. The results have shown that jet impingement is capable of delivering high transfer rates with lower cooling cost rates compared to current industry conventional techniques. Water is efficiently used in hybrid jet impingement because evaporative energy is absorbed directly from the surface instead of cooling air to near wet-bulb temperature. / Master of Science / Array jet impingement cooling experiments were conducted on a triangular grooved surface with the surface at a constant temperature. Results showed that jet impingement can provide high transfer rates with lower rates of cooling cost in comparison to contemporary conventional techniques in the industry. Experiments on the triangular grooved surfaces were performed at dry and wet surface conditions. Under the dry conditions, the objective is to characterize the resulting heat transfer under varying operational conditions such as jet speed, array orifice diameter, array to surface stand-off distance, and flow open area percentage. Results from the triangular surface when dry showed less improvement in heat transfer than the rectangular grooved surface. A hybrid cooling technique approach was proposed and developed by using air jets impinging on a triangular grooved surface with the grooves containing water. The approach is being suggested and experimentally tested for its viability as an alternative to thermoelectric power plant cooling towers. Convection heat and mass transfer coefficients were experimentally measured for different wet coverage of the surface. Results showed that the hybrid jet impingement produced high heat flux levels at low jet velocities and flow rates. The highest heat transfer was consistently found with a 50% coverage of the surface. Hybrid jet impingement showed an improvement up to 500% in heat transfer as compared to jet impingement on a dry grooved surface.

Array jet impingement cooling experiments were performed on flat and grooved surfaces with the surface at a constant temperature. For the flat surface, power and temperature measurements were performed to obtain convection coefficients under a wide range of operating conditions such as jet speed, orifice to surface stand-of distance, and open area percentage. Cooling performance (CP) was calculated as the ratio between heat transfer and fan power. An empirical model was developed to predict jet impingement heat transfer taking into account the entrainment effects. Experimental results showed that jet impingement can provide high transfer rates with lower rates of cooling cost in comparison to contemporary conventional techniques in the industry. CP values over 279 were measured which are significantly higher than the standard values of 70 to 95 in current technology. The model enhanced prediction accuracy by taking into account the entrainment effects; an effect that is rarely considered in the literature. Experiments on the grooved surfaces were performed at dry and wet surface conditions. Under dry conditions, results showed 10%~55% improvement in heat transfer when compared to the flat surface. Improvement percentage tends to be higher at wider gaps between the array of orifices and the grooved surface. An improvement of 30%~40% was observed when increasing Re either by increasing orifice diameter or jet speed. Similar improvement was observed at higher flow open area percentages. No significant improvement in heat transfer resulted from decreasing the size of the grooves from 3.56mm to 2.54mm. Similarly, no noticeable change in heat transfer resulted from changing the relative position of the jets striking the surface at the top of the grooves to the bottom of the grooves. Deeper grooves with twice the depth gave statistically similar average heat transfer coefficients as shallower grooves. Under wet conditions, a hybrid cooling technique approach was proposed by using air jets impinging on a grooved surface with the grooves containing water. The approached is proposed and evaluated experimentally for its feasibility as an alternative for cooling towers of thermoelectric power plants. Convection heat and mass transfer coefficients were measured experimentally using the heat mass transfer analogy. Results showed that hybrid jet impingement provided high magnitudes of heat flux at low jet speeds and flow rates. High coefficients of performance CP > 3000, and heat fluxes > 8,000W/m2 were observed. Hybrid jet impingement showed 500% improvement as compared to jet impingement on a dry flat surface. CP values of hybrid jet impingement is 600% to 1,500% more as compared to performance of air-cooled condensers and wet cooling towers. Water use for hybrid jet impingement cooling is efficient since evaporation energy is absorbed from the surface directly instead of cooling air to near wet-bulb temperature. / PHD / This thesis explored the possibility of using air jets on the outside surface of a device that is used to condense steam. An experiment apparatus was used to imitate the conditions of steam condensation in the lab. A flat metallic surface was heated by placing an electric heater beneath it. The metallic surface was cooled using air jets coming out of orifices situated above the hot metallic surface. A fan, connected to an electric motor, was used to create the air jets. The amount of heat transfer was measured by measuring the electric power the heater consumed. This measured power was compared to the power needed to run the fan. The ratio of heat transfer to fan power is called the coefficient of performance CP. The CP values of more than 200 were obtained when air jets were used meaning that we need one kilowatt of mechanical power to remove 200 kilowatts of heat. This CP value is 300% more than the current technology used in the industry where CP ranges from 70 to 90. This means that we can build very efficient steam condensers for power plants. This type of condensers that uses air jets allows the power plant to be efficient and to be able to increase the amount of power generated without extra cost. Further enhancement of the CP can be achieved by making the hot surface grooved instead of flat with the grooves containing water. Air jets, coming out of orifices situated above the grooved surface, were used for cooling. The CP values of more than 3,000 were obtained when air jets were used with wet grooved surface. This CP values is 1,500% more than the current technology used in the industry. This type of condensers that uses air jets on wet grooves allows the power plant to be efficient and to be able to tremendously increase the amount of power generated without extra power and water costs.