MM SCIENCE JOURNAL I 2022 I MARCH 5522 USE OF HOLOGRAPHIC INTERFEROMETRY FOR MONITORING HEAT TRANSFER ELENA PIVARCIOVA Technical University in Zvolen, Faculty of Technology, Department of Manufacturing and Automation Technology, Zvolen, Slovakia DOI: 10.17973/MMSJ.2022_03_2022007 e-mail to corresponding author: pivarciova@tuzvo.sk In the paper is described possibility of holographic interferometry in research of heat transfer above samples. Experiments with utilization of this method enable to explain many of actions going in environment and in phase interface between material and this environment in transport of heat. The presented experiments are aimed at verifying the possibilities of safe thermal loading of wood using with metal protective plate. KEYWORDS Heat transfer, wood, holographic interferometry 1 INTRODUCTION Wood is a natural material that has a very wide range of uses. Due to its advantageous physical and aesthetic properties, it is used both indoors and outdoors. It is often necessary to install various electrical devices, instruments and appliances on a wooden surface, e.g. in wooden houses, wooden tiles, furniture parts, etc. During the operation of electrical equipment, heat is generated and wood is a flammable material and this means the risk of ignition or damage to the wooden base or entire buildings. According to STN EN 13501-1, it is possible to classify individual types of wood into three classes of flammability groups according to the reaction to fire according to the table: Degree of flammability Class Kind of wood hardly flammable C oak and beech wood, sawdust boards ... moderately flammable D, E spruce, fir and pine wood, chipboard, cork parquet ... easily flammable F poplar wood ... Table 1. Degrees of wood flammability When installing electrical wiring (sockets, switches, junction boxes ...) and appliances (lights, heat sources ...) it is necessary to separate these objects from the flammable wood surface with a sufficiently large air gap or non-flammable pad over the entire contact area. Many authors study heat transfer by wood [Zhang 2013, Deliiski 2016, Younsi 2007]. To investigate the effect of heat load and heat field distribution on the surface and over combustible material, we performed experiments using wooden test specimens without the use of a protective device and with a protective metal pad on the side of the heat source. We used the method of holographic interferometry to visualize the thermal field [Brodnianska 2019]. 2 HEAT TRANSFER The issue of heat transfer is a current topic in terms of energy savings, finances, and a positive approach to the environment. Heat transfer between substances or their particles occurs in presence of a temperature difference [Pavelek 2009]. Heat transfer by conduction takes place in bodies (solids) and in stationary fluids (liquids or gases). It takes place by transfer of molecular energy between substances or their particles which come into contact and have different temperatures. The molecules do not move, they just oscillate and transfer energy to a cooler surface. The heat flux density decreases in the direction of decreasing temperature. The heat transfer by conduction through a simple planar wall is shown in Figure 1. Figure 1. Heat transfer by conduction through a simple planar wall – basic scheme [Brodnianska 2018] Q ̇ – heat flux [W] passing through a planar wall, λ – thermal conductivity coefficient [W.m –1 .K –1 ], S – heat exchange surface [m 2 ], Tp1, Tp2 – wall surface temperatures [K], δ – wall thickness [m] Calculation of the heat flux density through a simple plane is based on Fourier's law: (1) The heat flow passing through a planar wall with a heat exchange surface S is calculated by: (2) Thermal resistance of the wall (resistance to heat conduction) can be calculated by equation: (3) The heat transfer through a planar wall, composed of n layers, is shown in Figure 2. Figure 2. Heat transfer by conduction through a planar wall composed of n layers [Brodnianska 2018] Q – heat flux [W] passing through a planar wall, λ1, λ2,… λn – thermal conductivity coefficients [Wm –1 .K –1 ], S – heat exchange surface [m 2 ], Tp1, Tp2,… Tp(n+1) – wall surface temperatures [K], δ1, δ2,… δn, – wall thicknesses [m]