Research in Professor Arun Mujumdar’s group in the Department of Mechanical Engineering focuses on various innovative drying techniques of highly heat-sensitive materials.

In multi-mode heat pump-assisted drying of heat-sensitive materials, experiments and mathematical models have been developed to study the potential of utilising multiple modes of heat input (e.g. conduction, radiation, microwave energy, etc.) simultaneously or sequentially to minimise the drying time of heat-sensitive materials without compromising quality. One of our aims is to reduce the investment and operating costs of heat pump dryers so that they can be used effectively in a wider range of industrial applications. Aside from drying kinetics, the change of quality of the product during drying has also been investigated.

In collaboration with Dr. K. Kumar of the Institute of High-Performance Computing, computational fluid dynamic modelling of pilot and full-scale spray dryers has been performed to evaluate the effects of numerous design parameters on spray dryer performance. Among the new concepts evaluated are: various spray chamber geometries, a non-conventional lantern chamber and horizontal spray dryers, low pressure operation for highly heat-sensitive materials, potential for the use of an ultrasonic atomizer, use of superheated steam as a drying medium, two-stage horizontal spray-fluid bed dryer, etc. Figure 1 shows the air flow pattern and particle trajectories in a non-conventional horizontal spray dryer chamber. Note that the dried and partially dried droplets fall down to the bottom of the horizontal spray dryer due to gravity; the latter can be dried more cost-effectively in a fluid bed drying stage along the chamber bottom wall. Currently an industrial scale spray dryer for coffee is being modelled.

Figure 1: Air flow pattern (top) and evaporating droplet trajectories (bottom) in a horizontal spray chamber.

The use of the tailpipe of a pulsed combustor as a spray dryer for highly heat-sensitive materials is a relatively new concept yet to be explored fully. Unlike the conventional spray dryer, a p ulsed combustion spray dryer (PCSD) does not require a separate atomizer or a blower for the drying medium. The liquid to be dried to produce fine powder is simply injected as a liquid stream into the unsteady highly turbulent, high-temperature exhaust from the pulse combustor. Oscillations of the flow promote intensive mixing and augmented heat and mass transfer between the dispersed and continuous phases (Figure 2, top). Drying times can be in the order of fractions of a second because of the small droplet size and the high temperature of the drying medium (Figure 2, bottom). Current research is aimed at developing computational fluid dynamics (CFD) based models to examine the drying kinetics, energy efficiency as well as acoustic emissions from such a dryer. Concurrently we are also examining the feasibility of designing and testing micro-scale pulsed combustors with unique combustion chamber design.

Figure 2: Gas flow pattern within a PCSD drying chamber oscillate during a pulse combustion cycle (0.254-0.266s, 81 Hz) (top), and droplet drying rate (bottom).