Today, billions of people are limited to using biomass-fueled traditional fires as a primary energy source for cooking and purification of drinking water. Reliance on open burning of biomass leads to exposure to high levels of household air pollution which has been estimated to claim over 1.5 million lives annually, disproportionately affecting women and children in low- to middle-income countries. Additionally, the widespread dependence on inefficiently burned biomass fuels contributes to climate change on a global scale through the emission of as much as 20% of climate-forcing carbonaceous aerosols worldwide. To address the severe health and environmental impacts associated with traditional biomass-fueled cooking practices, researchers have been working to design and disseminate affordable cleaner-burning biomass cookstoves to vulnerable communities. Using a combination of computational and experimental approaches, hundreds of designs have been developed and guiding principles have been established. One of the most promising of these is the use of forced draft injection into the combustion chamber to improve both heat transfer and combustion efficiency. To contribute to this active area of research, this thesis will present the use of computational fluid dynamics to simulate the thermal and emissions performance of two unique configurations of forced draft biomass cookstoves.
In the first manuscript, a biomass cookstove with a forced draft primary air accessory is modeled and compared to experimental data produced following standard Water Boiling Test procedure. The study includes numerical and experimental results for a range of parametric variations corresponding to increasing primary air mass flow rates supplied to the combustion chamber. Thermal efficiency, as well as emissions metrics for carbon monoxide, carbon dioxide, and particulate matter are used to assess the performance of the affordable stand-alone primary forced draft accessory used in this study. For this configuration, improvements in thermal efficiency above 10% relative to the natural draft configuration are achievable, with corresponding PM emissions reductions resulting from each forced draft case. The simulation results show strong agreement with thermal efficiencies recovered in repeated ISO Water Boiling Tests.
The second manuscript presents computational fluid dynamics simulations of forced secondary air in a biomass cookstove. In this work, an array of ten unique simulations are used to understand how combinations of secondary air injection height and injection angle affect thermal efficiency and combustion performance and are compared to the results for a natural draft cookstove simulation. Air injection height and injection angle have a strong effect on the thermal efficiency of the cookstove, with the optimal combination of height and angle being 5 cm, and 90 degrees from the natural flow direction respectively. The CO2/CO emissions ratio predicted numerically suggest there is a design trade-off between thermal efficiency and emissions reduction.
Ultimately, the experimental and computational results show that integrating forced draft in either primary or secondary air configuration can lead to improvements in a range of performance metrics including thermal efficiency and emissions rates of CO and PM2.5. The goal of this thesis is to further the development and dissemination of inexpensive, user-friendly cookstove designs that help to make cooking with biomass fuel a sustainable and healthier option for the 40% of the world’s population who relies on it. Specifically, this work aims to provide cookstove designers with physical insights regarding forced draft biomass cookstoves which eventually might lessen the enormous health and environmental burden caused by traditional cooking practices.