接触式超声强化热泵干燥苹果片的干燥特性

Drying Characteristics of Apple Slices during Contact Ultrasound Reinforced Heat Pump Drying

为研究接触式超声对热泵干燥的强化效应,在热泵干燥机内安装了一套超声波装置,并以苹果片为研究对象,进行接触式超声强化热泵干燥试验,研究超声波功率、干燥温度以及切片厚度对苹果片干燥特性的影响。结果表明：将物料放在超声辐射盘上进行热泵干燥强化,有利于加快物料内部传质过程;随着超声功率和温度的增加以及厚度的减小,物料所需干燥时间逐渐缩短,平均干燥速率逐渐增大;超声对干燥速率的影响随着物料含水率的降低而减弱;在温度较低及物料较薄时,接触式超声的强化效果较好,但其对干燥速率的影响随着温度升高及物料变厚而有所下降;有效水分扩散系数的数值范围为1.333×10-10~1.651×10-9m2/s,且随着超声功率及温度的升高而增大;经过接触式超声处理的苹果片,其组织结构中的孔洞明显增多与扩张,在60 W超声功率作用下还形成了较多微细孔洞,从而有利于物料内部水分迁徙与扩散。将接触式超声技术用于热泵干燥过程的强化,可有效提高热泵干燥速率,缩短物料干燥时间。

In order to investigate the enhancing effect of contact ultrasound treatment on conventional heat pump drying,an ultrasound device was installed in the heat pump dryer, and contact ultrasound reinforced heat pump drying of apple slices was carried out to study the effects of ultrasound power,drying temperature and sample thickness on drying characteristics. The results showed that the internal mass transfer process of samples could be enhanced by putting samples on the ultrasound radiation board directly during heat pump drying. Drying time was shortened and drying rate was improved with the increases of ultrasound power and drying temperature and the decrease of sample thickness. The influence of ultrasound treatment on drying rate was decreased with the reduction of moisture content. Obviously strengthened effect of contact ultrasound on drying rate could be observed at low temperature and thin thickness,while it was reduced with the increases of drying temperature and thickness. The values of effective moisture diffusivity（ Deff） ranged from 1. 333 × 10- 10m2/ s to 1. 651 × 10- 9m2/ s,and it was increased at high ultrasound power and drying temperature. After the contact ultrasound treatment,the numbers and sizes of the tiny holes inside apple slice were improved. With ultrasound power of 60 W,many micro-tunnels were even produced in the organizational structure of apple slices,which was also beneficial to enhance internal moisture diffusion. Therefore,contact ultrasound technology is an effective method to increase the drying rate of heat pump drying and shorten drying time obviously as well.