INVESTIGATIONS ON THERMAL RADIATIVE CHARACTERISTICS IN A LAB SCALE FURNACE: EFFECT OF ADDITION OF NANOPARTICLES TO LPG COMBUSTION Khalid Waheed 1,2 , Seung Wook Baek 1 , Irfan Javed 1,2 , Yupiter Kristiyanto 1 1 Korea Advance Institute of Science and Technology (KAIST) KAIST 291 Daehak-ro (373-1 Guseong-dong), Yuseong-gu, Daejeon 305-701, South Korea 2 Pakistan Institute of Engineering and Applied Sciences (PIEAS) P.O. Nilore, Islamabad, Pakistan Khalid.waheed.sh@gmail.com, swbaek@kaist.ac.kr, ijkasuri@gmail.com ABSTRACT Use of nanoparticles in thermal applications have attained large interest in recent years because of their improved heat transfer characteristics. Effect of nanoparticle addition on radiative heat transfer characteristics from gaseous fuel combustion has been studied in this research. Radiative heat fluxes (RHFs) are considered to play a very important role in improving the thermal efficiency of furnaces by controlling heat transfer from the flames to the furnace wall. Among many contributors, presence of particles (soot and fly ash) contributes to larger percentage of radiative heat flux. However, in gaseous fuels combustion these kinds of particles are usually not generated or produced in low concentrations resulting in lower contribution of radiative heat transfer. Current research work is focused to investigate the effect of addition of combustible carbon black particle and noncombustible metal oxide nanoparticles on radiative heat transfer capabilities of LPG combustion. For this purpose, a furnace was designed with a vertical downward- propagating flame configuration to avoid any sedimentation of solid particles in the furnace. Diffusion flame was generated by providing separate supplies of fuel and oxidant. Noncombustible and combustible nanoparticles suspended in water in dilute concentrations were introduced to combustion chamber through a capillary placed at the center of fuel pipe. Concentration of nanoparticles in water suspensions was varied from 0.1 wt. % to 0.50 wt. % to avoid choking of feeding capillary. Temperature data inside furnace was obtained and significantly lower peak temperatures were observed. Heat flux data was recorded at the furnace wall and heat fluxes was significantly influenced by varying nanoparticle nanoparticles concentration. An overall increase in RHF was observed with maximum increase of 28% obtained with highest concentration of noncombustible nanoparticles. Higher contribution of RHF led to substantial increase in total heat flux. INTRODUCTION Nanofluids or engineered NPs laden fluids, gathered a great attention of researchers in recent years. Initially, they were solely employed as heat transfer fluids, but recently, due to discovery of enhanced characteristics of NPs, the nanofluids also found a broad range of applications in several areas of life, as discussed in several review papers [1-6]. Nanofluids are primarily useful in enhancement in heat transfer, higher critical heat flux and improvement in mass transfer [7]. Although the basic mechanism, explaining the unique properties of nanofluids has several limitations and uncertainties [3], their practical applications are unrestricted and widely popular. The current paper attempts to discuss for the first time, the effect of alumina nanofluids on radiative heat flux in LPG combustion. In any combustion process of fossil fuels, radiative heat transfer plays an important role in the efficient conversion of chemical to thermal energy, leading to the maximum possible extraction of heat out of a process. Contribution of radiative heat transfer to the total heat transfer can vary from 20-35% in diesel engines [8] to 95% in coal fired furnaces [9, 10], and thus cannot be neglected. A large volume of research articles [11, 12] and reference books [13-16] are available for explanation of radiative heat transfer in combustion phenomena in various application ranging from small engine to mammoth industrial furnaces. However, most of the references provide a detailed discussion only on the heat transfer process itself, with minimal or no comments on how to control such transfers effectively depending upon the end application. Many systems, such as, power plant furnaces, cracking furnaces, steel production and reheating furnaces require a high contribution of radiative heat to yield higher thermal efficiencies. Except for soot particles generated as a result of combustion, most of the two phase thermal radiations result from micron sized particles of coal and ash. By contrast, in gaseous fuel combustion, where micron sized coal and ash particles are absent and soot generated is also in small fraction, the radiative heat transfer contribution becomes significantly lower. The concentrations of particulate matter can be increased by seeding some nanoparticles to gaseous fuel flames. The effect of addition of nanoparticles to gaseous flame can profoundly increase the RHF contributions by increasing particulate matter within the furnace. However, to the best of our knowledge, no experimental research work has been performed so far to understand the effects of NPs on radiative heat transfer (transport of thermal radiation) in a combustion system. The use of NPs in combustion system is not so straight forward. Addition of nanofluids/nanoparticles in combustion process may result in a highly complex scenario of radiative heat transfer mechanism. To this end, temperature difference between hot combustion gases and nanoparticles may exist, along with changing absorption and scattering properties of hot combustion environment. In addition, there are considerable limitations on their practical applications in industrial furnaces. The three major drawbacks in this regard are cost feasibility, deposition of particulate matter on heat transfer surfaces and particulate emission. The use of NPs would not be much costly if we are able to understand how they affect the radiative properties. The deposition of NPs can pose significant impact on the working life and efficiency of convective heat transfer zone by forming a virtual cover on it. Before using NPs in furnaces it is important to analyze the efficiency of whole system, especially the possible decline in convection section efficiency