1 3 Heat Mass Transfer DOI 10.1007/s00231-015-1644-x ORIGINAL Experimental investigation on the effect of nanofluid on the thermal performance of symmetric sintered U shaped heat pipe Meysam Nazarimanesh 1 · Tooraj Yousefi 2 · Mehdi Ashjaee 3 Received: 7 August 2014 / Accepted: 19 July 2015 © Springer-Verlag Berlin Heidelberg 2015 1883 (W/m 2 ·°K). Also, due to unexpected changes in con- centration of 1000 ppm, the existence of an optimized con- centration for the silver nanofluid in this heat pipe with this geometry has been clear. List of symbols ΔT Difference temperature (°C) Q Load heat (W) R Thermal resistance (°C/W ) δ Partial derivative L Length (m) D Diameter (m) U Overall heat transfer coefficient (W/m 2 ·°K) 1 Introduction Heat pipes, in the last two decades, have been the center of attention as highly efficient tools which are used in transferring heat. Recent studies on the Heat Pipes have been considered as far as two general approaches for ris- ing heat transfer capacity and decreasing thermal resistance [1–8]. The first approach is optimizing the structure of heat pipes by altering their physical shape. The second method is performing investigations on more beneficial working fluids that are appropriate for heat pipes structures. These fluids, possessing certain features, are considered as highly conductive. In recent years, the use of heat pipes has experienced a dramatic increase in cooling electronic equipment. Both maximum overall Heat Transfer Coefficient (HTC) and minimum thermal resistance are significant parameters in heat pipes’ thermal performance. In order to improve these two parameters, nanoparticles are combined with the base fluid resulting in the formation of fluid that is called Abstract In this study, the impact of Entrance Power and Silver nanofluid concentration (with base fluid ethanol and DI-water) on heat pipe thermal performance are con- sidered. In order to reach the aim a U-shaped sintered heat pipe is utilized which causes occupied space to decline. The length of the heat pipe is 135 mm in each branch. On account of recognition the effect of working fluid on heat pipe thermal performance, thermal resistance and overall heat transfer coefficient in base working fluid and nano- colloidal silver are measured in the shape of thermosyphon. The working fluid is with volume percentages of 70 ethanol and 30 distilled water. The average size pertaining to the nanoparticle applied is 40 nm. In addition, the influences of nanofluid concentrations are measured by comparing three concentrations 0.001, 0.005, 0.1 vol%. The range of entrance power is from 10 to 40 W and the temperature of coolant has been changed from 20 to 40 °C. The results of the experiment indicate that by increasing entrance power, the temperatures of the condenser, evaporator and work- ing temperature experience a rise. Furthermore, this causes a decrease of thermal resistance and an increase of over- all heat transfer coefficient. A comparison of three con- centrations reveals that in concentration of 50 ppm, ther- mal resistance compared to the base fluid has decreased to 42.26 % and overall heat transfer coefficient has gone up to * Meysam Nazarimanesh nazarimanesh.meysam64@gmail.com 1 Mechanical and Industrial Engineering Department, Azad University of Tehran South branch, Tehran, Iran 2 Department of Mechanical Engineering, Faculty of Engineering, Razi University, Kermanshah, Iran 3 Department of Mechanical Engineering, Faculty of Engineering, University of Tehran, Tehran, Iran