Computer model of a domestic wood burning heater : a thesis presented in fulfilment of the requirements for the degree of Master of Engineering in Chemical Technology at Massey University

Abstract

Between April 2003 and April 2004 a project, funded by Technology New Zealand, was undertaken to develop a computer model of a wood burning heater for use at Applied Research Services Ltd. Applied Research Services Ltd is a science and engineering research company that specialises in the testing of wood burning heaters. The computer model will be owned by Applied Research Services Ltd and will be used to improve the design of their customers' heaters so that they may pass the particulate emissions and efficiency standards of AS/NZS 4013:1999. The computer model used the software program, Engineering Equation Solver as a platform to solve the model equations. EES was particularly easy to use and more emphasis was able to he placed on the actual modelling. The final model included over eight hundred variables and equations. It included radiant, convective and conductive heat flows, over thirty heat balances, Arrhenious based rate expressions and many empirical equations derived from experiments and data acquired at Applied Research Services Ltd. At the beginning of this project the objective was for the model to match the test results to within 10%. This has been met for the tests on the high airflow setting where the model error is 4% for flue temperature, 8% for heater output and 16% for flue oxygen. Unfortunately on low airflow setting, the model does not reach this target with model errors of 18% for flue temperature, 25% for heat output and 13% for flue temperature. The excellent results for the high flow setting are partially attributed to the use of calibration factors. The calibration factors model the processes in wood combustion that could not be modelled by this project, due to lack of time and resources. Some of these factors are the proportion of air that flows onto the charcoal ember bed or logs, radiation shape factor changes due to firebox geometry, convection heat transfer coefficients changing with turbulence. The calibration of the model only has to be completed once for each heater. The reason why the model does not work as well on low airflow setting is that with less airflow the proportion of air to the charcoal bed opposed to the logs would decrease, therefore decreasing the burn-rate. This model can he used to determine the changes to a heater's performance from changes to air inlet areas, insulation type and thickness, wetback size, baffle size, primary vs secondary air, air bypass ratio and door size. The model provides all the results that are obtained from an emissions test plus extra information such as the amount of excess air, smoke conversion in each combustion zone, flame temperatures and distribution of heat output. The smoke conversions for each combustion zone are particularly helpful in diagnosing where problems with the combustion occur. The reasons for incomplete combustion, lack of temperature or oxygen, can be found and fixed by increasing either insulation or air areas. The model can be used by Applied Research Services Ltd to improve heater designs. For the short term this will involve the author working as a part-time consultant. The project could be built on by another student by using CFD modelling for the sections of the wood burning process not modelled by this model and adding a graphical user interface to make the model easier to use