Effect of pipe flattening on pressure drop in vapor core and thermal characteristic of miniature round and flat-shape heat pipe with sintered fiber wick

•Numerical model for thermal resistance has been verified with the experimental results. Two different methods are used to evaluate the pressure drop and thermal resistance, i.e., ESDU79012 and Darcy-Weisbach equation.•The thermal resistance in vapor core needs to be considered in the miniature heat pipes.•Effect of pipe flattening on the pressure drop and thermal resistance in vapor core region is experimentally and numerically investigated.•The flattening of heat pipe caused the smaller space of vapor core, therefore the working fluid in vapor phase could not easily transfer heat. The pressure drop in vapor core region is enlarged which leads to increase of thermal resistance associated with vapor flow.•The normalized parameter (h′v/hv) is proposed to represent the apperance of vapor core after pipe flattening which can be used to suggest a desirable normal region of flattened heat pipe with the sintered fiber wick. The effect of pipe flattening on pressure drop in vapor core and also heat transfer characteristic of the sintered fiber heat pipes has been investigated. The original diameter of heat pipes were 2 mm and 3 mm, respectively, with the effective length of 100 mm. Thermal performance of the flat-shaped heat pipe with the sintered fiber bundle wick was experimentally investigated under a horizontal orientation in this study. The proper fiber bundle wicks were fabricated at porosity of 0.54, and firmly filled within internal heat pipe wall by the sintering process. The heat pipes were pressed into two final thickness by the flattened process which results in vapor core minimizing. The numerical method was established in modified forms to represent the related wick-heat pipe design parameters, for example, wick thickness, wick porosity, wall thickness, flattened width and height, corresponding to thermal performance of fiber wick heat pipes. The numerical analysis for pressure drop was only focused on vapor core region. The vapor flow was assumed to be laminar and incompressible. It is found that when the heat pipe was flattened, the pressure drop increased. The flattening of heat pipe caused the smaller space of vapor core, therefore the working fluid in vapor phase could not easily transfer heat. In this case, the critical final thickness of 0.45 mm (for original diameter of 2 mm) which the vapor core area nearly approaches to zero, has an effect on the high value of thermal resistance considered in vapor core (z5). In addition, the overall therm