Computational Modeling of a Biomass Micro-gasifier Cookstove

Computational Modeling of a Biomass Micro-gasifier Cookstove

Asranna, A.

Benediks, B.J. (mentor)
Van der Sluis, P. (mentor)
Pourquie, M.J.B.M. (mentor)
De Jong, W. (mentor)

Mechanical, Maritime and Materials Engineering

Process and Energy

2015-09-30

The World Health Organization (WHO) states that indoor air pollution resulting from inefficient burning of biomass in traditional cookstoves is a major health hazard affecting around 2.7 billion people globally. With the use of biomass for cooking energy expected to grow in the coming decades, efficient energy conversion technologies are an urgent need. The Philips woodstove based on the principle of micro-gasification of biomass is one such technology having the lowest reported emissions among wood burning cookstoves. The stove is based on experimental design and gaps exist in understanding the physics involved. Integration of simulations into the design process is expected fill these gaps and enable the characterization of key design parameters. A computational fluid dynamics based simulation model is developed for this purpose to investigate the fluid flows and heat transfer in the secondary combustion zone of the micro-gasifier stove. A three dimensional model of the stove is built using COMSOL Multiphysics. Combustion is modelled as a volumetric heat source with uniform heat generation. With appropriate boundary conditions a close approximation of the stove operating conditions is obtained and convergence achieved. The results demonstrate that the model is partially successful in simulating the flaming mode operation of the stove. The flow pattern is in agreement with visual observations. The temperature field inside the combustion chamber suffers because of the uniform heat generation assumption. The highest temperatures occur around regions of flow stagnation which is not representative of the actual operating conditions. The temperature values outside the combustion chamber are more reasonable, however still inaccurate. Including all the design details is expected to return more agreeable results. It has been realized that the discrepancy in the temperature field is because the secondary air flow rate specified in the model is only 43% of the experimental value. By specifying a secondary air flow rate closer to the experimental value, a closer approximation of the operating conditions is expected. A mixing analysis is carried out for two different heights of the fuel bed using passive scalars. The recirculation zone is found to have a significant impact on the mixing of the two streams. The simulations predict that at higher fuel bed heights the recirculation zone is restricted in space leading to poor mixing of the air and fuel stream. A validation study of the mesh used is conducted to ascertain its accuracy. A high Reynolds number jet is simulated and the results are compared with experimental values. The centerline velocity decay obtained using the mesh in regions close to nozzle exit is within 10% of the experimental values. The lateral velocity predictions are 20% lower than the experimental values. Since the distances in the computational domain do not extend beyond x=15D, the mesh used is expected to return accurate results. In its current state, the model is a good tool for flow visualization and understanding the broad qualitative trends. For comprehensively characterizing the design parameters of the stove, a better definition of the heat source term and reactive flow modelling is necessary.

Cookstoves
Biomass
Gasifier stove
Philips woodstove

http://resolver.tudelft.nl/uuid:09244c43-76db-4575-816f-73619b22b50d

Student theses

master thesis

(c) 2015 Asranna, A.