Diffuse Thermal Radiation Focusing Device

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1 Diffuse Thermal Radiation Focusing Device Design Team Keegan Deppe, Ernest Kabuye Kevin Liu, Jeff Masters, Guy Shechter Design Advisor Prof. Gregory Kowalski Abstract The object of this Capstone Design Project is to develop a Diffuse Thermal Radiation Focusing Device. This product consists of a diffuse thermal radiation source encapsulated by a paraboloid-shaped reflector. The reflector will collect the radiation emitted from the source and focus that radiation into a collimated beam. Radiation only directly from the focal point of the paraboloid will become collimated upon reflection. The challenge of this project is that the source will realistically be larger than a point, thus there will be radiation emitted in all directions from its surface and not the focal point. This project focuses on using the engineering design approach to develop a prototype that will collimate the source generated radiation and to provide a test platform and methodology to evaluate and improve upon the efficiency of the collimated beam. Paraboloid reflector Collimated energy Diffuse radiation source Figure 1 Two dimensional side view For additional information, contact Gregory Kowalski at gkowal@coe.neu.edu

2 The Need for Project This project provides a green During the winter, it is a common practice for motorists to warm up solution to melting ice on their automobiles on a daily basis in order to defrost the windshield. windshields, stairs, Not only is it wasteful to idle the car in this manner, but it is also plane wings, dams etc. detrimental to the environment. The catalytic converter burns off hydrocarbons in the exhaust, but it does not operate properly until it reaches high temperatures. Until this operating temperature is reached, several pounds of CO 2 are released into the atmosphere. The Design Project Objectives and Requirement Develop a prototype that will Design Objectives capture and collimate the The objective of this project is to design a device that will focus radiation from a diffuse heat the diffuse thermal radiation generated from a designed source into a source and implement a collimated beam of radiation with a circular cross-sectional area. The measurement system to evaluate design considerations include the geometries of the source and the prototype. parabolic reflector, as well as materials selection. These considerations will be addressed in order to optimize the device efficiency, focusing as much radiation into a collimated beam as possible. A collimated beam can be focused and directed for heating purposes, and this heating method could possibly be more efficient than a laser. Design Requirements The most challenging aspect of this design is to overcome the major radiation losses. A diffuse point source, a theoretical point with a radius of zero, will emit radiation in all directions. If this point source were placed at the focal point of the paraboloid reflector, then all the radiation emitted from that point would become collimated upon reflection. Realistically, it is not possible to have a point source with a radius of zero. Instead, the source would have a radius greater than zero, resulting in a surface area made up of multiple off-axis points. If this realistic source were placed at the focal point, only a small percentage of radiation would become collimated because only a small percentage of the radiation coming from these off-axis points are radiating collinear to the focal point. Figure 2 shows a red point source in the center. As the point source increases in radius to the gray circle, the red rays indicate the radiation emitted from the point and the black rays indicate the radiation emitted in all directions from each point of Figure 2 - Radiation rays the surface.

3 Design Concepts considered The main goal is to design a prototype that collimates as much radiation from a diffuse heat source as possible. The initial efficiency metric was 90%, meaning 90% of the radiation from the source would be collimated in the beam. It was theoretically proven that the device could only achieve 30%, so the design goal has been refocused to develop a measurement system intended for the evaluation of the product efficiency. This measurement platform allows the beam to be manipulated and it will provide a method to evaluate this manipulation. Alternate design concepts Source Material considered different source The initial source material considered is nichrome wire. Nichrome geometries, variations in is an incandescent material that emits radiation when heated to high reflector geometry, as well as temperatures. It was chosen over tungsten, aluminum and steel because materials selection. it has a higher electrical resistivity, a higher radiative emissivity, and it is inexpensive. Source Design The source design consists of a coil of nichrome wire embedded within a ceramic hemisphere source that will be held in place by mineral-insulated cables. The purpose for the ceramic is to allow for a more uniform distribution of the radiation. Alternative designs considered were solid spherical sources and bare wire. Reflector Material Figure 3 - Nichrome embedded in Initial design ideas considered polished stainless steel and radiant a ceramic. The nichrome leads are barrier. The ideal material will have a high reflectivity and a low visible on the right. emissivity. Radiant barrier is a material developed by NASA to reflect radiant heat rather than slow the heat transfer process. It has an emissivity and reflectivity of 0.1 and 0.9, respectively, but its ability to reflect radiant radiation made it the optimal choice over stainless steel. Reflector Design The parabola shape was the intrinsic concept for the reflector. All radiation coming from the focal point of the parabola will reflect off the inside surface and become collimated. Thus, the diffuse source will be positioned at the focal point. Since the source radius is greater than zero, the off-axis points on its surface will emit radiation that does not get collimated, shown by the black arrows in Figure 2.

4 Recommended Design Concept A coil of nichrome wire embedded in a ceramic will generate radiation in response to electrical current. A paraboloid reflector coated with radiant Design Description This product consists of a diffuse thermal radiation source and a reflector. The source is constructed of nichrome wire embedded in a ceramic hemisphere. Nichrome was selected by using the following equation: barrier will collimate the radiation. The measurement where is the source emissivity. This equation shows that the source system to evaluate the product performance will measure temperature change, due to the incident energy of the beam, of a must have a high electrical resistance and radiative emissivity, as previously mentioned. The reflector will be a rapid-prototyped paraboloid that follows the general equation: copper plate inside an emulated blackbody. where a is the focal length of the parabola. This equation allows for the evaluation of different sized paraboloids based on focal length. The optimal focal length is determined to be m because it yields the most desirable paraboloid size. The reflector will be painted with a radiant barrier coating to reflect the radiation. The source will be suspended at the focal point of the reflector by mineral insulated cables and oriented such that the curved surface faces the inside of the reflector. Analytical Investigations Central investigations, regarding theoretical performance, considered the diameters of the source, the parabola, and intensity of the beam. Maximizing the beam intensity is ideal, but this maximum is dependent on the operating efficiency of the device, which is dependent on the view factor from the heat source to the paraboloid reflector. This view factor was evaluated to be. Thus, the intensity of the beam is governed by where is the emissivity of nichrome, is the Stefan-Boltzmann constant, and is the surface temperature of the source. This equation shows that the intensity is dependent of the source temperature. Increasing the source diameter allows for increased

5 power input, but also requires a larger parabola diameter. Leveraging these interdependencies, while maintaining a portable device, yields a beam intensity of approximately 2 watts. Experimental Investigations The measurement system consists of a copper plate mounted inside a box designed to emulate a blackbody. Four thermocouples are attached to the plate at varying positions, and two thermocouples will measure the ambient temperature. Using this system, the temperature change in the plate can be used to determine the beam efficiency. From there, the measurement system can be used to evaluate different manipulations of the beam. The main function of the testing platform is for the optimization of the beam intensity. Figure 4 Measurement system Initial analyses show that approximately 30% of the radiation from a spherical source at the focal point of a paraboloid will become collimated. Experimental investigations considered varying the distance between the device and copper plate and adding a radiation shield to the flat side of the source to redirect the rays. Key Advantages of Recommended Concept The key advantage of this project is that it is environmentally friendly compared to other processes used for defrosting windshields, such as idling the car or using de-icing spray. This solution conserves radiation and is not detrimental to the environment. Financial Issues This product is intended to be The prototype cost is approximated around $200. This cost mass produced. The advertised includes materials for the source, reflector, and measurement system. commercial cost is expected The commercial cost for this product will be about the same as the to be $200. prototype. If a lens system were added to improve the system and produce a sheet of radiation, then the commercial cost would increase to $1200-$1500. Recommended Improvements A second phase is recommended The current phase of research and development provides the test to reduce the product size and to platform and methodology for solving this problem. A second phase add a lens system to improve the of R&D is recommended in order to include a lens system that would product efficiency. manipulate the beam to improve the system efficiency. A second phase is also needed in order to reduce the size of the device for portability.