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
Blanching is an essential step before processing of agricultural products as it can inactivate enzymes that cause undesirable changes.In current work the effects of superheated steam impingement blanching(SSIB)time an...Blanching is an essential step before processing of agricultural products as it can inactivate enzymes that cause undesirable changes.In current work the effects of superheated steam impingement blanching(SSIB)time and temperature on drying characteristics and red pigments content of line pepper were investigated.Results showed that after a 3-min SSIB pretreatment at 110℃ the pepper epidermis covered with wax coat was damaged.In addition,the drying time was extensively decreased and the loss of red pigments of dried products was reduced.Results showed that the whole drying process took place in the falling rate period,which indicated that diffusion was the dominant physical mechanism governing moisture movement in the samples.Therefore,the second Fick’s law of diffusion was used to determine the effective moisture diffusivity(Deff)of line pepper,which increased from 1.193×10^-10 to 3.128×10^-10 m^2/s with increasing of the drying temperature and air velocity.The drying activation energy(Ea)of pretreated samples was 34.31 kJ/mol,which decreased by 3% compared with the non-pretreatment group.The findings of this work indicate that SSIB is very promising pretreatment technique as it not only enhances drying process but also improves bioactive substance preservation of red pepper.展开更多
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.
基金the National Natural Science Foundation of China(Grant No.31360399)for the financial support.
文摘Blanching is an essential step before processing of agricultural products as it can inactivate enzymes that cause undesirable changes.In current work the effects of superheated steam impingement blanching(SSIB)time and temperature on drying characteristics and red pigments content of line pepper were investigated.Results showed that after a 3-min SSIB pretreatment at 110℃ the pepper epidermis covered with wax coat was damaged.In addition,the drying time was extensively decreased and the loss of red pigments of dried products was reduced.Results showed that the whole drying process took place in the falling rate period,which indicated that diffusion was the dominant physical mechanism governing moisture movement in the samples.Therefore,the second Fick’s law of diffusion was used to determine the effective moisture diffusivity(Deff)of line pepper,which increased from 1.193×10^-10 to 3.128×10^-10 m^2/s with increasing of the drying temperature and air velocity.The drying activation energy(Ea)of pretreated samples was 34.31 kJ/mol,which decreased by 3% compared with the non-pretreatment group.The findings of this work indicate that SSIB is very promising pretreatment technique as it not only enhances drying process but also improves bioactive substance preservation of red pepper.
