Research On The Raindrops Size Distribution And Its Effect On The Cooling Performance And Pressure Drop Of The Rain Zone In The Large Wet Cooling Tower

The rain zone is one of the most important part in the wet cooling tower.In the conventional wet cooling tower(CWCT),the heat and mass transfer between air and water happens firstly in the rain zone,the cooling performance and the ventilation subsequent is directly related to the air flow characteristic in the rain zone.In the wet cooling tower with water collecting devices(WCTWCDs),although the scope of the rain zone is reduced obviously,it is still the first region where the heat and mass transfer happens and the last area before the air flows into the water collecting zone.As the dimension of the wet cooling tower is getting oversized,the scale of the rain zone is getting larger.The heat and mass transfer and the air flow characteristic in the rain zone have an increasing impact on the cooling performance of the tower.So the study of the cooling tower becomes more and more important to the wet cooling tower.However,the countless amounts and the complicated motions of the raindrops make the rain zone study rather difficult.Currently,studies of the rain zone is relative rare compared to other parts of the rain zone,and nearly all the studies make the rain zone oversimplified.The oversimplification on the rain zone is sure to have great influence on the cooling performance research.It is therefore very necessary to conduct intensive study on the rain zone of the wet cooling tower.The effect to the cooling tower induced by the rain zone can be divided into two parts,that is,the thermal performance and the pressure drop performance.These two parts are all closely related to the raindrops size distribution.And the raindrops size distribution is always the difficulty in the rain zone research.In view of this,the theoretical analysis of the raindrops size distribution is conducted firstly,the mechanism of the raindrops motion such as the deformation,the breakup and the collision is analyzed,and the impact to the raindrops size distribution caused by the raindrops motion is also studied.Then the field test of the raindrops measurement in the rain zone is conducted based on the theoretical analysis,the scattering techniques is employed first time in the cooling tower research.The characteristics of the raindrops size distributions in the rain zone under both windless and crosswind conditions are obtained,which can clarify the validity of theoretical analysis results.On the basis of the experiment results,the numerical model which can describe the raindrops size distributions characteristics precisely is then established.Then the rain zone is studied intensively in both numerical and experimental way.The accurate results of the thermal and pressure drop performance of the rain zone is revealed accordingly.The impact introduced by the raindrops size distribution on the thermal and pressure drop performance is also discussed.Since the wet cooling tower can be divided into CWCT and WCTWCDs,the rain zone research except the theoretical analysis are also divided into two parts.Moreover,in the WCTWCDs the WCDs are installed under the fill,leaving the rain zone scale reduced and making the air flow field at the air inlet changed.So as the supplement of the rain zone research,the air flow field in the WCDs and its impact on the cooling performance of the WCTWCDs is finally studied systematically.The main efforts of the thesis are as follows,(1)This thesis starts with the theoretical analysis of the raindrops size distribution,the motion mechanism of raindrops and its impact on the size distribution is firstly explored.It is revealed from the theoretical level that the raindrops size distribution inside the rain zone is not totally disordered and random.(2)Based on the theoretical analysis the field test of the raindrops measurement in the rain zone is then conducted,the disdrometer which is often used in the meteorology is employed,a unique stretchable jib arm which is often used in film and television production is designed by the author to make sure the test can be carried on smoothly.The results of the experiment are as follows.For a fixed positions in the wet cooling tower,although the raindrops size distributions at different test moments show some small differences,they still share a similar distributions in general.At the same height in a certain CWCT,although there exist differences in the raindrops size distributions in different test positions,the raindrops still share the same size distribution since the falling duration is nearly the same in the windless conditions.That is to say,the raindrops size distribution will not change along the radial and circumferential directions.However,when the height of the test positions change,the falling duration of the raindrops at different positions is different,making the raindrops size distribution change along the height.For different CWCTs,the height of the rain zone is the main influence factor to the raindrops size distribution.That is to say,the raindrops size distribution at different positions in different CWCTs is similar as long as the falling distance is same.For theWCTWCDs,since the rain zone scope is reduced significantly,the interaction among the raindrops and its impact on the size distribution is slight,so the raindrops at the initial diameter account for the largest proportion.In the WCTWCDs,the raindrops size distributions are not affected by the crosswind at all,but in the CWCT it is the absolutely opposite situation.In the CWCT,the interaction among the raindrops is extremely intense under the strong crosswind condition.In such circumstance,the falling duration is not the key influence factor to the raindrops size distribution,the raindrops at different height share a similar size distribution,and the raindrops at small diameter account for the largest proportion.(3)The test results are sorted out by the statistical analysis,and the optimal size distribution is derived to represent the raindrops size distribution in the rain zone of the wet cooling tower.Then the cumulative distribution is used to make the size distribution equational.In this way,the research results of the raindrops size distribution can be applied into the cooling tower study conveniently,making the raindrops size distribution practicable and realistic meaningful.(4)The rain zone performance of the wet cooling tower is studied intensively.As to the thermal performance,the rain zone of is the most important heat and mass transfer area second to the fill zone in the CWCT.However,in the WCTWCDs,the rain zone scope is reduced significant,making the heat transfer occurred in the rain zone reduced substantially.Furthermore,the proportion of the raindrops at the small diameter in the WCTWCDs account for more than that in the CWCT,making the specific surface area of the raindrops in the WCTWCDs smaller than that in the CWCT.So for the same rain zone scale,the heat transfer capability in the WCTWCDs rain zone is weaker than that in the CWCT in terms of raindrops size distribution.As to the resistance performance,the pressure drop in the rain zone of the CWCT accounts for the largest portion of that in the whole tower.The resistance to the air casued by the raindrops when it flowing through the rain zone has a great impact on the performance of the CWCT.However,in the WCTWCDs,the pressure drop portion of the rain zone drops sharply.Moreover,because of the slight raindrops interaction,the raindrops amount in the rain zone in WCTWCDs is small.Therefore,for the same rain zone scale,the pressure drop caused by the raindrops in the WCTWCDs is smaller than that in the CWCT in terms of raindrops amount.For the WCTWCDs,the pressure drop of the fill zone and the spray zone barely changes with the reduction of the rain zone scope contrast to the CWCT with the same cooling area,whereas the temperature drop of the fill zone rises significantly.It means that the rain zone scope,the raindrops amount and the raindrops size distribution have great impact on the performance of the wet cooling tower.And it also emphasizes the importance of the rain zone again on the other side.(5)The new correlations for the Merkel number and the pressure drop calculation of the rain zone are developed and validated by the dimensional analysis and the non-linear regression analysis.It can be used to calculate the rain zone pressure drop and analyze the rain zone thermal performance credibly.The accurate analysis on the rain zone can be realized in consequence.(6)The air flow field inside the water collecting devices(WCDs)and its influence to the WCTWCDs compared to the CWCT are studied as the supplement of the rain zone research.The results indicate that the change in the air flow direction has the significant impact on the air flow field inside the WCDs.When the inclined angle is changing,the air pressure fluctuation caused by the abrupt turning at the WCDs junction has stronger influence to the air flow field than the vortex on the inclined slab.The air pressure fluctuation at the inlet is the main influence to the air flow field in the WCDs when the aisle spacing is changing.The thermal performance of the fill zone in the WCTWCDs is distinctly different from that in the CWCD,which is the cooling capacity is strongest at the inner fill and weakest at the outer fill.In the WCTWCDS,the air flow field at the air inlet is different from that in the CWCD since the rain zone is substituted by the WCDs,which makes the cooling performance of the WCTWCDs different from the CWCD.The air resistance of the air inlet in the WCTWCDs is smaller than that in the CWCD with the same structure feature.Moreover,the air is deflected by the inclined slab when flowing through the WCDs,making the velocity increased.So the air mass flow rate of the WCTWCDs is higher that of the CWCD and the cooling performance of the WCTWCDs is greater than that of the CWCD.