In the present work, a micro combustor for thermophotovoltaic (TPV) devices is proposed which is included a U-shaped microtube in a box with a secondary fluid in space between U-shaped microtube and box walls. By utilizing the three-dimensional CFD model, combustion characteristics of the premixed lean hydrogen-air mixture in the present micro combustor are studied numerically with detailed chemistry and transport taking into account heat transfer through the wall. The results show that the establishment of secondary flows and better preheating in the curved tubes is caused the flammability limits to be at least four times in comparison with straight tubes. Combustion characteristics are studied for different parameters, namely inlet velocity, wall thermal conductivity, heat loss conditions, tube curvature and number of U-shaped tubes. By increasing inlet velocity up to 16 m/s, flame front moves toward downstream, the maximum temperature is increased and rate of reactions is intensified. The highest efficiency for thermophotovoltaic devices is calculated at 4 m/s inlet velocity. A wall thermal conductivity of 3 Wpm K creates a better condition for flame stability. It is shown that the secondary fluid with higher thermal diffusivity could improve combustion characteristics. Energy conversion efficiency (n), emitter efficiency (nRad) and total energy conversion efficiency of the TPV (ntotal) is utilized to investigate the simulated cases for the present geometry. This type of micro combustor is well fitted for thermophotovoltaic applications. (C) 2017 Elsevier Ltd. All rights reserved.