Contents lists available at ScienceDirect Solar Energy journal homepage: www.elsevier.com/locate/solener Experimental analysis on free convection effect using two different thermal performance enhancers in absorber tube of a forced circulation flat plate solar water heater K. Balaji a, ⁎ , A. Idrish Khan b , P. Ganesh Kumar c , S. Iniyan a , Ranko Goic d a Department of Architecture and Planning, IIT Roorkee, India b Department of Mechanical Engineering, Thiagarajar College of Engineering, Madurai, India c Department of Mechanical Engineering, Anna University, Chennai, India d Faculty of Electrical Engineering, Mechanical Engineering and Naval Architecture, University of Split, Croatia ARTICLE INFO Keywords: Grashof number Rayleigh number Richardson number Wall heat flux Flat plate solar water heater Free convection ABSTRACT The objective of this experimental work is to compare the convection effect of the flat plate solar water heater (FPSWH), both with and without thermal performance enhancer, and to find the optimum mode of convection in the FPSWH. Two different types of thermal performance enhancers have been used, viz., rod and tube, and they were frictionally contacted with a solar absorber tube. The water flow rate in the FPSWH was varied from 0.008 kg/s to 0.025 kg/s. The Grashof number, Rayleigh number and Richardson number, which are based on solar irradiance, have been considered in this study for evaluating the effects of different modes of convection in a FPSWH. Based on the analysis, which a mixed convection heat transfer phenomena occurred in the FPSWH with the free convection domination. The plain copper tube FPSWH has significantly higher Grashof number, Rayleigh number and Richardson number in flows were compared to rod and tube thermal performance en- hancer fitted collector. 1. Introduction Flat solar water plate heater (FPSWH) is a device used for the conversion of solar energy in to thermal energy (hot water). Increase in application of hot water in the built environment (domestic and in- dustry) has become a daily occurrence. Further, service sectors such as hotels, restaurants, hostels and others need hot water in large quan- tities. FPSWH plays a vital in for the hot water production due to simplicity in construction and environment friendly nature (Esen and Yuksel, 2013). FPSWH construction and improved techniques used for performance enhancement have been presented in detail in earlier research studies (Aleksiejuk et al., 2018; Sadhishkumar and Balusamy, 2014). The major components of flat plate collectors are the absorber tube, the absorber plate, storage tank and glass. Different parameters relate to perfor- mance of the flat plate collector. They include irradiation, ambient temperature, Heat Transfer Fluid (HTF) inlet temperature, collector angle, wind speed, HTF (Esen, 2004; Esen and Esen, 2005), absorber tube configuration, and type of material in each component. Passive and active techniques are used for the increase in the FPSWH thermal performance. Reviews have reported the significant role played by passive technique in overall energy and thermal performance en- hancement without any addition of parasitic electrical energy (García et al., 2018; Naphon, 2006). Absorber tube profile modification is one of the passive convective heat transfer enhancement techniques used in FPSWHs. A detailed literature study on hydraulic and thermal perfor- mance of different types of thermal performance enhancers in the ab- sorber tube such as twisted tape, integral fin, coil spring, swirl flow generator and compound has already been presented (Ji et al., 2015; Liu and Sakr, 2013; Varun et al., 2016). The mode of convection using twisted tape, conical ridges and coil-spring wire was studied in straight tube, and the results were interpolated for the FPSWH (Hobbi and Siddiqui, 2009). The author concluded that the thermal performance enhancement devices in the laminar flow FPSWH were ineffective owing to buoyancy effect. Over the past decade, extensive studies have been conducted on FPSWH using various types of thermal performance enhancement de- vices of various types in the absorber tube (Ananth and Jaisankar, https://doi.org/10.1016/j.solener.2019.04.089 Received 21 January 2019; Received in revised form 3 April 2019; Accepted 29 April 2019 ⁎ Corresponding author. E-mail addresses: mr.kbalaj@gmail.com (K. Balaji), idrish92@gmail.com (A. Idrish Khan), selgan.cad@gmail.com (P. Ganesh Kumar), iniyan777@hotmail.com (S. Iniyan), rgoic@fesb.hr (R. Goic). Solar Energy 185 (2019) 445–454 0038-092X/ © 2019 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved. T