Heat and mass transfer in forced convective drying of biomass
using non-intrusive temperature and mass measurements
Project coordinator(s)
Prof. Darina B. Murray
Email: dmurray@tcd.ie
Tel: +353 1 896 1129
Dr. Tim Persoons
Email: tim.persoons@tcd.ie
Tel: +353 1 896 1936
Mr. Eoin Fanning
DescriptionConvective drying is an essential production process in a wide a range of industries such as food processing, textiles, pharmaceuticals and is also a common process in farming and refinement of raw materials. In recent years, convective drying has become an increasingly important process in the treatment of biofuels and a common heat use for combined heat and power (CHP) plants, particularly biomass fueled CHP plants. High moisture content (MC) in Biofuels (e.g. willow chips, miscanthus) reduces the efficency of combustion and the net calorific value of fuel gas produced in gasification, increases microbial growth during storage and also increases CO and volatile emissions during combustion.
This research explores the transient heat and mass transfer from an initially water-saturated porous cube under forced convective drying conditions. The evolution of mass transfer rate and surface temperature of a porous body in a convective drying process shows three main stages: A pre-heating stage, an isothermal evaporation stage and a falling-rate period. The influence of key parameters such as air temperature, velocity and relative humidity on the average surface temperature and drying rate of a porous body has been explored extensively for different geometries and flow configurations.
The present study investigates the simultaneous time evolution of mass transfer rate and local surface and core temperature of a porous cube inside a rectangular flow channel for a range of air flow temperatures and Reynolds numbers. The instantaneous mass transfer rate is measured using a precision mass balance. Local surface temperature distribution is measured using a thermal imaging camera via an infrared transparent window.
The time evolution of mass transfer rate and surface temperature confirms the three main stages in the drying process, in good agreement with previous studies. Thermal images show a uniform wet-bulb temperature across the side of the cube which is constant during the isothermal evaporation period. In this quasi steady state period, an average Nusselt number between 11.4 = Nu = 17.7 is calculated for a Reynolds numbers range of 350 = Re = 700. As the falling-rate period begins, the temperature distribution becomes non-uniform and approaches the temperature of the surroundings. As expected, this transition first occurs at the edges of the cube. The project aims to include local temperature measurements of the air flow surrounding the cube by using laser-induced fluorescence (LIF) thermometry and model more closely the capillary-porous structure of biomass mateirals such as willow.
Funding BodyHEA funded Graduate Research Education Program (GREP) in Engineering
Images
