| In the total energy consumption, the energy consumption of buildings and telecommunication centers is considerable. As a special building, a telecommunication center needs to be cooled for a whole year. And the heat dissipation for a sealed electric cabin with dense electronic components is huge. But the required environmental conditions, such as temperature, humidity and cleanliness, are very strict to make sure the devices work well. As an effective heat transfer unit and a good cooling technique, a two-phase closed thermosyphon is able to make full use of outdoor ambient energy, reduces the air conditioning energy consumption, and isolates the outdoor humidity, dust or acid gas. However, the key points of a two-phase closed thermosyphon, like heat transfer efficiency, working fluid match and environmental characteristic, still need to be studied further in the room temperature from 0 to 60 degrees. According to the above situations, the following jobs were done:(1) The numerical simulation and experimental investigation of the thermal property, Merit Number, entrainment heat transfer limit and boiling heat transfer limit were carried out for different working fluids and its compositions with different mass fractions. It is pointed that, the working fluids with bigger Merit Number values are priorities. The working fluids, R152a, R290, R32 and R410A, show better performances among the fifteen working fluids. Due to the toxicity and flammability of the working fluid, compositions with two or three ingredients are feasible. For R32-base compositions, the mass fraction of R32 should be higher, and is 90% for compositions with two ingredients. For the R32/R134a/R245fa composition, the mass fractions should be 40%, 30% and 30%, respectively. For the R32/R290/R245fa composition, the mass fractions should be 30%, 40% and 30%, respectively. It is suggested that some other ingredient should be added into the base working fluid to meet the actual needs. And some working fluids with different proportions can be charged for different pipe rows and different temperature zones.(2) The simulation heat transfer models of a two-phase closed thermosyphon and a heat exchanger were built up. The heat transfer process, flow characteristics and pressure variations were studied theoretically and experimentally. It is found that, for a thermosyphon, the temperature difference of indoor and outdoor should be less than 10 degrees for a better heat transfer performance. And the length ratio between evaporation section and a thermosyphon should be under 0.2 or over 0.5. The ratio between evaporation section length and inner diameter should be not more than 35, and the heat flux should be less than 5 W/m2. For a thermosyphon heat exchanger, the effect of indoor and outdoor temperature difference is over that of working temperature. It is recommended that lower outdoor ambient temperature, lower facing air velocity and higher temperature difference will be better. The changing rules of theoretical and experimental results keep the same, and the error is not over 15%.(3) The single wall carbon nano tubes (SWNTs) were applied in a thermosyphon base working fluid to enhance the heat transfer with some surfactant polyethylene glycol (PEG) with molar mass 400 and the heat transfer characteristics were studied. It is found that, the improvement of PEG 400 for a thermosyphon is less than 3%, which can be ignored. The improvement of SWNTs is over 40%, which is higher with lower pressure when under the 30 degrees working temperature. And the suitable mass fraction is not more than 0.005%. So the SWNTs with not over 0.005% mass fraction are suggested to be added into a base fluid for a better thermal performance under the 30 degrees working temperature.(4) A new separated thermosyphon heat exchanging system with a gas pump was proposed to improve a conventional separated thermosyphon heat exchanger and a prototype was developed. It is found that, the efficiency, heat transfer rate and energy efficiency ratio (EER) increase by 47.8%, 130% and 29%, respectively. It shows that the gas pump can improve the system performance effectively.(5) The simulations of a telecommunication center for national typical cities in five climatic zones were carried out. The numerical and experimental researches of the building envelope heat dissipation, the heat load and energy consumption of an air conditioner and a thermosyphon heat exchanger were completed under different working conditions and different building envelopes. The quantitive results were obtained. And a union operation mode was presented for an air conditioner-thermosyphon heat exchanger system. It is found that, the building envelope with the lowest yearly air conditioning energy consumption among the six type building envelopes is 370 mm brick wall in Harbin, 240 mm brick wall in Beijing, 180 mm brick wall in Shanghai, Kunming and Guangzhou. The heat dissipation through the building envelope takes about 19.5% of the total communication equipment heat load. And the air conditioner energy consumption is 3 to 4 kW h for 24 hours. The air conditioning energy consumption dereases by about 3.6% as the indoor set temperature increases by 1 degree. And the indoor set temperature is recommended to be 27 degrees. In the union operation mode, the thermosyphon heat exchanger is prior to the air conditioner. The energy consumption decreases to 41% of the air conditioning energy consumption when a thermosyphon heat exchanger works. And the whole year energy saving ratio reaches to 56%.(6) The thermal performances of a thermosyphon spreader with different working fluids in a sealed electric cabin were studied experimentally. It is found that, the highest temperature in the electic cabin is not more than 65 degrees when the indoor and outdoor temperature difference is less than 20 degrees. The indoor and outdoor temperature difference remains in the range of 11 to 17 degrees, which indicates that it is very suitable to the situation with 15 degrees temperature difference. The working fluids, R152a and R22, show better performance than the other fluids, R12 and R134a. Further, the Merit Number variation of the working fluid R152a is smooth, the average is higher, and the optimum temperature zone is wider.The above investigations supply a theoretical reference for improving the heat transfer of a two-phase closed thermosyphon, especially for the working fluid and its compositions matching and single wall carbon nano tubes (SWNTs) heat transfer enhancement. A thermosyphon heat exchanger is matched in a new ambient energy utilization field, the air conditioning energy consumption of a telecommunication center and a sealed electric cabin. In addition, the proposal of a new separated thermosyphon heat exchanger with a gas pump solves the problems of a conventional separated thermosyphon heat exchanger in engeering applications. In generally, the reseaching jobs consider the environmental protection of working fluid alternatives while meeting the heat dissipation and reduction of energy consumption. And it also supports the energy saving and emission reduction of the telecommunication industry. The energy saving is remarkable in present engineering projects.
|
PDF Full Text Request