Thermal management of portable biomedical vaporisers

This thesis presents the thermal characterisation of a gas p owered, portable device used for the vaporisation of herbal materials for biomedical inh alation. The main aims of this research are to characterise the thermal behaviour of a cham ber assembly inside the device where the material is heated to its vaporisation temperatur e, o quantify the minimum power required to heat the herbal material, to provide designers o f such chambers with tools which allow the prediction of the power requirements for heating t he material, and finally, to validate these tools. The physical and thermal material properties of two herbal m aterials lavender and chamomile were measured, for use in power requirement calculati ons. In order to determine thermal efficiency of the current industrial state of the art in vaporisa tion of herbal materials, a commercial portable vaporiser was thermally charac te ised. These measurements showed that only 2% of the heat generated in the device was sup plied to the target material. This poor efficiency was attributed to unnecessarily large components wi thin the device and hence unnecessarily large thermal capacitance and larg e su face area for convective heat loss to ambient. In order to determine the minimum power requirements which c ould potentially be achieved to heat the desired volume of herbal material, further analy sis was restricted to just the material and the chamber assembly in which it is contained. Two initial design tools were developed, which allow the prediction of spatial and tempor al temperature distribution in a centrally heated, two dimensional annulus of material sub jected to convective boundary conditions. The first provides non-dimensional charts for d etermination of temperature. The second provides an interactive design method, which sol ve the governing equations for unsteady conduction in radial co-ordinates based on a ra nge of variables input by the user, which calculates the temperature distribution and po wer requirements for heating the two dimensional annulus of material from ambient to its vapo risation temperature. The flow path of fluid inside the chamber assembly of the vapori sation device and through the porous medium during inhalation was investigated exper imentally using Particle Image Velocimetry (PIV) and refractive index matching. These mea surements were used primarily for validation of Computational Fluid Dynamics (CFD) mo dels. Vector maps are also presented to provide an understanding of the flow structure i nside the chamber, where fluid enters the porous medium downstream of a sharp bend. A flatten ed velocity profile was found after fluid travelled through a short distance of low pe rm ability porous material. The temperature distributions in the chamber assembly and p orous material when heat is applied from ambient and during inhalation were measured ex perimentally; the results of which were used for validation of CFD predictions. Computat ional models were used for the overall prediction of the power required to heat the mate rial initially and reheat after inhalation. Finally, a number of power profiles are presente d which express the relationship between power supply limits and the allowable material he t-up time. This provides a guideline when selecting the type and size of power supply in future designs of the vaporisation device. iii