Investigation Of Surface Roughness Effects On Micro-channel Flow And Heat Transfer Characteristics

Owing to the rapid development in manufacturing technology,millions of circuits can be integrated into one miniaturized chip.However,the consequent increase in generated heat flux requires more efficient cooling approach to maintain the operation of chips.Thus,micro-channels are preferred to be used in many electronic devices due to their inherent ability to offer larger surface area per unit volume,which increases the heat transfer rate and prolongs the life span of these devices.To meet the demands of super-high heat flux dissipation,new designs are developed for further enhancing the heat transfer behavior of micro-channel heat exchangers.Several innovative structures have already been devised on channel walls to improve the thermal performance.However,this rise in heat transfer is usually accompanied with the cost of high pressure drops.It is important to precisely understand the factors affecting the thermal behavior and pressure drop of micro-channel.To improve the heat transfer performance,some studies have been reported for micro-channels with roughness elements.Yet,detailed flow characteristics of rough micro-channel are the pre-requisites for applications.This work focuses on the numerical analysis and detailed investigation of micro-channel with different shapes and distribution of surface roughness,thermal-hydraulic conditions,and dimensional parameters.The constricted flow parameter scheme is considered to interpret the roughness effects on micro-channel flow.It is confirmed by present study that original constricted model can predict the frictional characteristics within the deviation of 15%by comparing it with experimental results in literature.Firstly,flow and heat transfer characteristics of smooth micro-channel are studied for low aspect ratios(?=0.012 to 0.12).The friction factor of the studied micro-channels can be precisely predicted through the laminar flow theory.The Nu of ?=0.02 is almost 12%higher than that of ?=0.12 at Re=400.New correlations are developed for the better estimations of Nu for both the air and water as working fluids in micro-channels.Secondly,the effect of surface roughness peaks(SRP)on laminar flow and heat transfer performances in rough micro-channels is analyzed.The variations of channel height,SRP height,pitch,and width,aligned and off-set SRP,and hybrid SRP in rectangular channels are studied.The maximum convective heat transfer depends on the optimum combination of channel size and roughness height.Furthermore,increasing SRP pitch has less impact on the heat transfer while increasing roughness width improves the performance index(?).Channel with hybrid SRP(r/r'=30/15 ?m)is found more efficient than the rough channels with respective constant SRP(r=30 ?m,r=15 ?m)(at Re_c ?300),which is mainly due to the lower f_c.However,at Re_c=400 the maximum value(77)is noted for r=15 ?m and ? increases with increasing Re_c for all the cases.Aligned peaks are more efficient than the off-set arrangement and the average rise of ? is 9%under present conditions.Relative roughness with varying parameters can be configured for practical purposes without compromising the compactness.Thirdly,micro-channels structured with rectangular surface roughness are numerically analyzed for thermal-hydraulic performances of laminar flow.The performances are compared between working fluids air and water,and under different heating walls and roughness configuration conditions at constricted Reynolds number(Re_c)of 50 to 250.The results show that both fc and Nuc of water are slightly higher than those of air.The ? of air increases with increasing Rec but the trend is opposite for water,and the average deviation between them is 11%.The convective heat transfer with one rough heated wall is almost 40%lower than that of two heated walls,whereas the f_c almost remains the same.The channel structured with regular surface roughness(constant roughness height)shows about 14%higher values of ? than those of the respective channel with hybrid surface roughness at Re_c=200.The difference of heat transfer performance is because of variable heated surface area and flow mixing.Fourthly,numerical simulations are performed to study the impact of constricted aspect ratio ac on the micro-channels behaviours in the presence of sinusoidal surface roughness.The considered channel height,absolute roughness height and constricted channel diameter are 250 ?m,30 ?m and 366 to 374 ?m,respectively.The convective heat transfer and overall thermal performance both improve with the increase in Re_c where all the ? values are higher than 1.The maximum value of the ? is noted up to 1.6 for the channel of ?_c=0.038(Rec=250)due to the lower corresponding values of f_c.The average rise of ? for the channel with ?_c=0.038 at Re_c=250 is almost 6%more than the other variations(?_c=0.021,?_c=0.015).Fifthly,thermal-hydraulic characteristics of 3D micro-channels are investigated with random surface roughness.These characteristics of rough micro-channels are found co-dependent on surface roughness(?),channel diameter and Rec.The correlations of fc and Nuc are developed for better estimations,whereas the density of surface roughness has less effect on the characteristics.The constricted flow model with average roughness height(ravg)predicts normalized friction factor closer to the existing experiment-based correlation than r_max.The f_c of micro-channel with r_avgis also found to obey laminar theory.The increase of inlet temperature improves the Nu_c.? is observed less than 1 only when ??5%at Re_c?100,while it is higher than 1 for the rest of the range of 2.2%???9%at Rec?250.Performance comparison of all the surface roughness shapes(SRP,Rectangular,Sinusoidal,and Random)is made by considering a common case(D_c=374 ?m,r_max=30 ?m).It is found that the overall performance of channels with surface roughness peaks and sinusoidal surface roughness are higher than the other rough channels.The maximum value of ? for the channel with surface roughness peaks is 1.07 at Re_c=50,while at Re_c=250,the maximum value of 1.28(?)is found for sinusoidal surface roughness.The difference of ? between them is not more than 10%(50 ? Re_c?250)within the considered range of parameters.It is resolved at the end that surface roughness effects cannot be ignored on micro-channel laminar flow.Also,the presence of surface roughness may have positive influence on overall thermal-hydraulic performance and can be an efficient way to improve the cooling performance.It is also worth mentioning that relative roughness cannot be the sole criteria for the performance identification and prediction of micro-channel,whereas it behaves in a different way with varying channel diameter,therefore related correlations are developed for precise estimations.Based on the present study and literature review,it is also observed that suitable correlations are needed for the prediction of convective heat transfer(Nusselt number)in the laminar flow range for rough micro-channel.Therefore,relevant correlations are developed precisely for the considered range of parameters.These findings significantly improve the knowledge to the existing literature related to the design of micro-channel.This work might also play a key role in designing and performance optimization of micro-channel based heat exchangers for electronic devices.