Thermal management of portable biomedical vaporisers

This thesis presents the thermal characterisation of a gas powered, portable device used for the vaporisation of herbal materials for biomedical inhalation. The main aims of this research are to characterise the thermal behaviour of a chamber assembly inside the device where thematerial is heated to its vaporisation temperature, to quantify theminimumpower required to heat the herbal material, to provide designers of such chambers with tools which allow the prediction of the power requirements for heating the material, and finally, to validate these tools. The physical and thermal material properties of two herbal materials - lavender and chamomile - were measured, for use in power requirement calculations. In order to determine thermal efficiency of the current industrial state of the art in vaporisation of herbal materials, a commercial portable vaporiser was thermally characterised. These measurements showed that only 2% of the heat generated in the device was supplied to the target material. This poor efficiency was attributed to unnecessarily large components within the device and hence unnecessarily large thermal capacitance and large surface area for convective heat loss to ambient. In order to determine theminimumpower requirements which could potentially be achieved to heat the desired volume of herbal material, further analysis 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 temporal temperature distribution in a centrally heated, two dimensional annulus of material subjected to convective boundary conditions. The first provides non-dimensional charts for determination of temperature. The second provides an interactive design method, which solves the governing equations for unsteady conduction in radial co-ordinates based on a range of variables input by the user, which calculates the temperature distribution and power requirements for heating the two dimensional annulus of material from ambient to its vaporisation temperature. The flow path of fluid inside the chamber assembly of the vaporisation device and through the porous medium during inhalation was investigated experimentally using Particle Image Velocimetry (PIV) and refractive index matching. These measurements were used primarily for validation of Computational Fluid Dynamics (CFD) models. Vector maps are also presented to provide an understanding of the flow structure inside the chamber, where fluid enters the porous medium downstream of a sharp bend. A flattened velocity profile was found after fluid travelled through a short distance of low permeability porous material. The temperature distributions in the chamber assembly and porous material when heat is applied from ambient and during inhalation were measured experimentally; the results of which were used for validation of CFD predictions. Computational models were used for the overall prediction of the power required to heat the material initially and reheat after inhalation. Finally, a number of power profiles are presented which express the relationship between power supply limits and the allowable material heat-up time. This provides a guideline when selecting the type and size of power supply in future designs of the vaporisation device.