Airside Flow And Heat Transfer Characteristics Of Cooling Deltas And The Heat Transfer Enhancement Mechanism Of Wedged Jet

With its energy and water saving advantages,the natural draft dry cooling units are mostly used as the main thermal power units in the large energy bases of "scenery,fire and storage integration" in central and western China.Heat exchangers at the cold end of cooling units are mostly in the form of cooling columns with four or six rows of tubes arranged in a triangular shape along the circumference of the indirect air-cooling tower.Cooling column,cooling triangle unit as the core components of the cooling tower,its heat transfer characteristics will directly affect the cooling performance of the whole tower.However,the heat transfer characteristics of the cooling delta units are closely related to the incoming air conditions in the cooling delta and are particularly influenced by the crosswind.Therefore,it is important to address the flow and heat transfer characteristics of the air side of the cooling delta of the natural draft dry cooling tower and its heat transfer enhancement mechanism to further improve the energy and water saving.In this paper,the air-side flow and heat transfer characteristics of the cooling delta are experimentally investigated for a 1000MW natural draft dry cooling unit,and the feasibility of optimizing the air flow field and air-side heat transfer characteristics of the cooling delta using a wedge-shaped gap radiator is indicated.Considering that the cooling column tube bundle in the natural draft dry cooling tower is fin-type radiator,its air-side thermal resistance has the greatest influence on the heat transfer coefficient between air and water,this paper assumes that the water-side flow rate of the cooling column tube bundle is constant and only focuses on the air-side flow heat transfer characteristics of the cooling delta.Through long-term monitoring of real-time parameters such as wind speed,wind direction and wall temperature at a number of measurement points in a typical cooling delta,combined with ambient meteorological conditions,the distribution regulations of crosswind at the scale of cooling columns in the cooling delta is derived.On this basis,the regulations obtained from the field tests was used to validate the numerical model.Numerical simulations were carried out for two cases of natural draft dry cooling towers with conventional radiators and with wedge gap radiators at different wind velocities to compare the flow heat transfer characteristics such as ventilation and outlet temperature of tower,and to propose a comprehensive outlet temperature to measure the cumulative effect of the wedge gap radiator on the long-term constant operation of the unit.The results show that under various wind velocities,the application of wedge-shaped gap radiators can significantly increase the tower’s ventilation and reduce the whole tower outlet temperature,with a comprehensive outlet temperature reduction of 0.49℃.At the same time,wedge-shaped gap radiators have a higher heat transfer coefficient and lower flow resistance coefficient,which can ensure the cooling performance of natural draft dry cooling tower under higher wind velocities.In view of the huge size of natural draft dry cooling tower and the difference in air inlet conditions of the cooling delta at different locations,five typical locations were selected to compare the flow heat transfer characteristics of usual radiators and wedge gap radiators at these locations and to clarify the mechanism of wedge gap radiators.The results show that the opening of the wedge gap allows the up cooling column to come into direct contact with the ambient air,rather than the secondary air already through the down-side cooling column,and therefore has a lower inlet air temperature,while the high velocity wedge jet increases the inlet air volume to the up cooling column,thus significantly improving the heat transfer performance of the up cooling column and hence the overall heat transfer performance of the radiator.In addition,this paper compares the effect of the wedge gap angle on the heat transfer performance.The results show that the wedge angle should not be too large or too small,the angle of 2.5°when the radiator performance reached the best.When the angle is too small the effect is not very obvious,when the angle is larger than 2.5° wedge-shaped gap radiator performance is no longer improved with the increase in the angle.