Heat Flux Characteristics of Asymmetrically Heated and Cooled Thermal Stimuli

The human thermal response system can be manipulated by the proper combination of applied hot and cold stimuli. Previous research has shown that a sensation of constant cooling can be perceived through the application of certain patterns on the skin. Here we focus on (1) exploring the heat flux characteristics of the thermal display through computer simulations, (2) testing a hypothesis about the relationship between thermal sensation and heat flux, and (3) examining modifications of the thermal display patterns to intensify thermal sensations. To characterize the heat flux patterns of the thermal display, finite element simulations were performed using ANSYS. Simulations were done in two parts: the first examined a small subregion between heating and cooling stimuli, and the second was a larger scale examination of the heat flux profile of the thermal display. It was observed that the heat flux profiles for all thermal patterns were approximately identical. A linear relationship is derived between simulation and experimental results. This relationship was then used to determine the theoretical thermal sensations to determine which are best suited for future physical experimentation on humans. INTRODUCTION The human sensory receptors detect external changes in the environment and transmit signals to the brain. The brain interprets these signals and responds accordingly. Well known receptors include: chemoreceptors, mechanoreceptor and thermoreceptors. Each of these receptors are responsible for ⇤A. Manasrah is currently affiliated with Al-Zaytoonah University of Jordan. specific interactions with the environment. Chemoreceptors and mechanoreceptor are responsible for senses such as taste, smell, and touch [1, 2]. Thermoreceptors are responsible for temperature sensation as well as the control and regulation of body temperature [3]. Thermoreceptors are located throughout the skin at various depths and in various concentrations. When a thermal stimulus is present on the skin, thermoreceptors begin to fire signals to the brain. When the brain receives these signals, it interprets them as either hot or cold and responds accordingly. The interpretations of thermal stimuli by the brain affect other areas of human perception as well. For example, skin temperature has an affect on vibrotactile perception [4]. Thermoreceptors are divided into warm and cold receptors [5] with cold receptors being more prevalent in the human skin than warm receptors at a ratio of thirty-to-one [6]. Additionally, warm and cold receptors respond to different rates of temperature change [7] and are activated at different ranges of temperatures. Warm thermoreceptors are active in the range from 30 C to 45 C. Cold thermoreceptors are active in the range of decreasing temperatures from 30 C to 18 C. Below 18 C and above 45 C, thermal sensation transitions to a feeling of pain which is transmitted through receptors called nocioceptors [8]. The sensation of pain has been shown to be directly related to the magnitude of the temperature difference even when hot and cold temperatures are well below the pain threshold [9]. As the area of thermal stimulation increases, the perceptible threshold decreases [7]. Kenshalo [7,10] showed that the rate of temperature change determines the perceivable thermal threshold. Additionally, 1 Copyright c 2017 by ASME thermoreceptors respond to decreasing temperatures much quicker than increasing temperatures [11, 12]. Manasrah et al. showed that a perceived sense of constant cooling [13] and constant heating [14] could be achieved through the use of asymmetrically heating and cooling thermal actuators. However, thermal perception is not based on absolute temperature but on temperature difference or heat flux. Therefore, the primary contribution of this paper is the development of a relationship between the thermal perception of the patterns used in the physical experiment and the heat flux values calculated from the simulations. BACKGROUND Heat Flux Heat flux is defined as the rate of heat transfer per unit area, i.e., W/m2. The equation for heat flux can be written as: