Heat exchangers on the inside of the muffler absorb heat from the exhaust as it flows through. The heat passes through exchangers that line the inside of the UAV’s aluminum skin to 2-in. square TEGs mounted on the outside of the UAV and finally passes through another row of heat exchangers to the open air. As the TEGs are exposed to the temperature difference between the hot inside exhaust air and the cool outside air, they generate electric current. The greater the temperature difference, the more current is generated.
The team modeled the thermal design of the system using Mentor Graphics FloTHERM computational fluid dynamics (CFD) 3D modeling software . They simulated airflow on the outside (cool air) and the hot exhaust inside to estimate the temperature difference, which enabled them to optimize the internal and external fins of the heat exchanger and the number and location of the TEGs.
They built engineering models of several likely EHTEG configurations and ran them on a test stand using the same 3W80xi engine and propeller that is used in a MLB Company Bat4 UAV. They validated the FloTHERM CFD models for a range of operating parameters that simulate flight conditions by carefully measuring temperatures, electrical output, and exhaust flow rates using a custom-built airflow test chamber. Exhaust gas composition for calculating mass flow and specific heat was derived from previous work [3, 4].
They started with the internal volume and length for the muffler recommended by the manufacturer that was required to make the system act as an efficient expansion chamber exhaust system for the two-stroke engine. These were used to develop the first half-symmetry CFD models that would determine the number of TEGs needed to optimize the electrical output with minimal weight. The model is symmetrical about a longitudinal vertical plane, so building a half-symmetry model reduced the number of elements down to 1.04 million cells with no loss in accuracy.
They kept the fin parameters of the internal and external heat exchangers relatively constant while varying the location of the heat exchangers and the placement of the TEGs. The goal was to maximize the temperature differential across each TEG to extract the most heat energy from the 455°C exhaust gas.
They modeled 13 configurations of heat exchangers and TEGs in the first optimization study. Then they tabulated the power output from each configuration and chose the best configuration. For example, when the heat exchangers were spread out over the full length of the muffler, the hot exhaust flow had difficulty reaching the first set of heat exchangers. The design was improved by placing all the heat exchangers close together toward the midsection of the muffler (Figure 2).
Figure 2: The positions of the external heat exchangers that resulted in the highest average power generation for the system.
After optimizing the placement of the TEGs, they found that the central fins on the interior heat exchangers disrupted the exhaust flow and the hot exhaust gas wasn’t reaching the front end of the muffler. But if they removed the center fins, the exhaust gas would not channel down the center and the exhaust pulse would not reach the front end of the muffler. So instead of removing the central fins, they placed them in a semicircular pattern (Figure 3). This configuration kept the exhaust pulse moving through the center of the muffler and it was able to curl back very symmetrically as the hot gas flowed back along the outsides and through the fins.
Figure 3: The interior heat exchanger with the fins in a semicircular shape in the center.
The interior heat exchangers decreased in temperature as the hot exhaust gas flowed from the front of the muffler through the fins to the rear of the muffler and then out of the vertical exhaust pipe. With FloTHERM simulation, the team was able to see the exhaust gas flow pattern for the heat exchanger with the center fins removed (Figure 4). The flow pattern is disrupted before the exhaust stream reaches the front end of the muffler.
Figure 4: Thermal simulation shows the exhaust gas flow pattern without fins in the center. The top image is temperature and the bottom is speed.
In comparison, the simulation showed that when the semicircular-shaped fins are present, the flow pattern retains its shape all the way to the front of the muffler (Figure 5). This comparison allowed them to optimize fin placement.
Figure 5: Thermal simulation shows the exhaust gas flow pattern with fins placed in a semicircle in the center. The top image is temperature and the bottom is speed.
Usually outside heat exchangers on TEGs are placed all in a line, which is a problem because the units toward the rear of the external airflow receive more preheated air than the heat exchangers that are upstream. This configuration reduces the delta-T across the heat exchanger and reduces the power output.
So they studied nine configurations to determine the optimum fin parameters for the external heat exchangers. Then they plotted the results against the total power generated by the TEGs. Although all of the custom heatsink designs they analyzed out-performed the stock heatsinks for total power generation, because of time and budget constraints, they used stock heatsinks for the first flight test model. (As a result, they lost about 11 W power output compared to the best possible option.)
The outside TEGs were arranged in four columns by two rows on each side of the muffler. The TEGs maintained a cool side temperature below 58.3°C with external air at 18°C and 22.3 m/s velocity, while keeping the hot side temperature below the maximum allowable temperature of 225°C (Figure 6). The outside heat exchanger temperature ranges from 26.1°C on the leading edge of the front fins to 58.1°C on the base plate next to the hottest TEG.
Figure 6: Cool-side temperatures of the outside TEGs.
Some of the first simulations demonstrated that the heat loss through all other surfaces of the muffler, other than the exterior heat exchangers, had to be minimized to maximize heat flow through the TEGs for maximum power generation. They used mineral wool sheet insulation for all the exposed surfaces to minimize the heat loss because it can withstand temperatures of more than 300°C and added little weight to the muffler assembly.
They were able to model the power output of the EHTEG system by summing the power contribution of each pair of TEG modules. By first calculating the hot side and cold side temperatures of each TEG pair, they could then use this data to compute the open circuit voltage. The harvesting and power conditioning circuitry matches the equivalent series resistance for maximum power transfer; thus, the voltage of the load resistance is exactly half the open circuit voltage. This data defines the power harvested per TEG pair. The results for the flight configuration are shown in Figure 7.
Figure 7: Single-side power output (W) for the flight configuration.