The paper presents an experimental investigation on enhanced heat transfer and pressure loss characteristics by using single, double, triple, and quadruple twisted-tape inserts in a round tube having a uniform heat-fl...The paper presents an experimental investigation on enhanced heat transfer and pressure loss characteristics by using single, double, triple, and quadruple twisted-tape inserts in a round tube having a uniform heat-fluxed wall. The investigation has been conducted in the heat exchanger tube inserted with various twisted-tape numbers for co- and counter-twist arrangements for the turbulent air flow, Reynolds number （Re） from 5300 to 24000. The typical single twisted-tape inserts at two twist ratios, y/w = 4 and 5, are used as the base case, while the other multiple twisted-tape inserts are aty/w = 4 only. The experimental results of heat transfer and pressure drop in terms of Nusselt number （Nu） and friction factor 00, respectively, reveal that Nu increases with the increment of Re and of twisted-tape number. The values of Nu for the inserted tube are in a range of 1.15-2.12 times that for the plain tube while f is 1.9-4.1 times. The thermal enhancement factor of the inserted tube under similar pumping power is evaluated and found to be above unity except for the single and the double co-twisted tapes. The quadruple counter-twisted tape insert provides the maximum thermal performance.展开更多
The article presents an experimental and numerical study on thermal performance enhancement in a constant heatfluxed square-duct inserted diagonally with 45° discrete V-finned tapes(DFT).The experiments were carr...The article presents an experimental and numerical study on thermal performance enhancement in a constant heatfluxed square-duct inserted diagonally with 45° discrete V-finned tapes(DFT).The experiments were carried out by varying the airflow rate through the tested square duct with DFT inserts for Reynolds number from 4000 to 25000.The effect of the DFT with V-tip pointing upstream at various relative fin heights and pitches on heat transfer and pressure drop characteristics was experimentally investigated.Both the heat transfer and pressure drop were presented in terms of Nusselt number and friction factor respectively.Several V-finned tape characteristics were introduced such as fin- to duct-height ratio or blockage ratio(R_B=e/H = 0.075,0.1,0.15 and 0.2),fin pitch to duct height ratio(R_P= P/H=0.5,1.0,1.5 and 2.0) and fin attack angle,α = 45°.The experimental results reveal that the heat transfer and friction factor values with DFT inserts increase with the increment of R_B but the decrease of R_P.The inserted square-duct at R_B = 0.2 and R_P = 0.5 provides the highest heat transfer and friction factor while the one with R_B= 0.1 and R_P= 1.5 yields the highest thermal performance.Also,a numerical simulation was conducted to investigate the flow structure and heat transfer mechanism inside the tested duct with DFT inserts.展开更多
The heat transfer performance of a mist/air jet impingement on a constant-heat flux surface was experimentally investigated.Two objectives were outlined in the current study.The first objective is to assess the effect...The heat transfer performance of a mist/air jet impingement on a constant-heat flux surface was experimentally investigated.Two objectives were outlined in the current study.The first objective is to assess the effects of mist/air volumetric flow rate ratio,impinging mode and heat flux on the heat transfer characteristics of free mist/air jet impingement.The second objective is to assess the effect of swirl flow induced by the spinning grinding wheel on the mist/air jet impingement,simulating the heat transfer process on a grinding work-piece surface subjected to the mist/air jet impingement.The results show that the addition of dilute water droplets to air flow results in significant heat transfer enhancement.Once the mist/air ratio is increased to a certain value,the increase of heat transfer with the mist/air ratio becomes slow.For a given mist/air ratio,as the increase of heat flux,the contribution of droplet evaporation to the overall heat transfer is weakened relatively,resulting in a decrease of heat transfer enhancement in comparison to the lower heat flux case.The heat transfer coefficient in the stagnation region for the oblique jet is much lower than the normal mist/air jet impingement,while in the region away from the stagnation,the local heat transfer coefficient for the oblique jet is higher than the normal jet.As regards as the mist/air jet impingement in the vicinity of grinding zone is concerned,when the jet impinging direction is consistent with the rotating direction of rotating disk,the swirl flow induced by the rotating disk could entrain more droplets to enter the jet impinging stagnation zone,which is beneficial to convective heat transfer enhancement.Furthermore,as the rotational speed of disk increases,the temperature deceases in impinging jet stagnation zone.展开更多
基金the Thailand Research Fund(TRF)(Grant No.Ph D/0143/2552)
文摘The paper presents an experimental investigation on enhanced heat transfer and pressure loss characteristics by using single, double, triple, and quadruple twisted-tape inserts in a round tube having a uniform heat-fluxed wall. The investigation has been conducted in the heat exchanger tube inserted with various twisted-tape numbers for co- and counter-twist arrangements for the turbulent air flow, Reynolds number （Re） from 5300 to 24000. The typical single twisted-tape inserts at two twist ratios, y/w = 4 and 5, are used as the base case, while the other multiple twisted-tape inserts are aty/w = 4 only. The experimental results of heat transfer and pressure drop in terms of Nusselt number （Nu） and friction factor 00, respectively, reveal that Nu increases with the increment of Re and of twisted-tape number. The values of Nu for the inserted tube are in a range of 1.15-2.12 times that for the plain tube while f is 1.9-4.1 times. The thermal enhancement factor of the inserted tube under similar pumping power is evaluated and found to be above unity except for the single and the double co-twisted tapes. The quadruple counter-twisted tape insert provides the maximum thermal performance.
基金Supported by the Energy Policy and Planning Office,Ministry of Energy,Thailand
文摘The article presents an experimental and numerical study on thermal performance enhancement in a constant heatfluxed square-duct inserted diagonally with 45° discrete V-finned tapes(DFT).The experiments were carried out by varying the airflow rate through the tested square duct with DFT inserts for Reynolds number from 4000 to 25000.The effect of the DFT with V-tip pointing upstream at various relative fin heights and pitches on heat transfer and pressure drop characteristics was experimentally investigated.Both the heat transfer and pressure drop were presented in terms of Nusselt number and friction factor respectively.Several V-finned tape characteristics were introduced such as fin- to duct-height ratio or blockage ratio(R_B=e/H = 0.075,0.1,0.15 and 0.2),fin pitch to duct height ratio(R_P= P/H=0.5,1.0,1.5 and 2.0) and fin attack angle,α = 45°.The experimental results reveal that the heat transfer and friction factor values with DFT inserts increase with the increment of R_B but the decrease of R_P.The inserted square-duct at R_B = 0.2 and R_P = 0.5 provides the highest heat transfer and friction factor while the one with R_B= 0.1 and R_P= 1.5 yields the highest thermal performance.Also,a numerical simulation was conducted to investigate the flow structure and heat transfer mechanism inside the tested duct with DFT inserts.
基金supported by the National Natural Science Foundation of China(Grant No.51076063)
文摘The heat transfer performance of a mist/air jet impingement on a constant-heat flux surface was experimentally investigated.Two objectives were outlined in the current study.The first objective is to assess the effects of mist/air volumetric flow rate ratio,impinging mode and heat flux on the heat transfer characteristics of free mist/air jet impingement.The second objective is to assess the effect of swirl flow induced by the spinning grinding wheel on the mist/air jet impingement,simulating the heat transfer process on a grinding work-piece surface subjected to the mist/air jet impingement.The results show that the addition of dilute water droplets to air flow results in significant heat transfer enhancement.Once the mist/air ratio is increased to a certain value,the increase of heat transfer with the mist/air ratio becomes slow.For a given mist/air ratio,as the increase of heat flux,the contribution of droplet evaporation to the overall heat transfer is weakened relatively,resulting in a decrease of heat transfer enhancement in comparison to the lower heat flux case.The heat transfer coefficient in the stagnation region for the oblique jet is much lower than the normal mist/air jet impingement,while in the region away from the stagnation,the local heat transfer coefficient for the oblique jet is higher than the normal jet.As regards as the mist/air jet impingement in the vicinity of grinding zone is concerned,when the jet impinging direction is consistent with the rotating direction of rotating disk,the swirl flow induced by the rotating disk could entrain more droplets to enter the jet impinging stagnation zone,which is beneficial to convective heat transfer enhancement.Furthermore,as the rotational speed of disk increases,the temperature deceases in impinging jet stagnation zone.
