A sensor element that consists of a thermally resistive layer made of nand p-type semiconductor elements amplifies the temperature gradient in the resistive layer that results from heat flow through the sensor. The thermoelectric array provides greater accuracy and sensitivity over a traditional thermopile arrangement for heat flux measurement. The thermoelectric sensor array is an adaptation of technology developed for generating electricity in radioisotope thermoelectric generators that operate with internal temperatures in excess of those at Venus. The technology is used in a different manner in that, instead of generating electrical power, it measures heat flow using a temperature differential output and a voltage output.
In operation, the sensor is deployed on a boom or arm that is attached to a Venus lander. During descent through the atmosphere, the sensor is exposed to the Venus environment because it resides on the exterior of the vehicle. At deployment, the sensor is likely to be at thermal equilibrium with the local environment. After the sensor contacts the ground surface, a current is passed through the thermoelectric elements of the sensor. This provides a cooling of the copper plate at the bottom of the sensor. The plate is heated by the flux coming from the Venus surface. The transient response of the sensor provides a direct correlated measurement of the heat flux at the surface. It is not necessary for the sensor to come back to equilibrium with the environment to make a heat flux measurement. This transient measurement technique allows for several heat flux measurements to be made with the sensor while the lander is operating.
This work was done by Michael T. Pauken, Suzanne E. Smrekar, Jean-Pierre Fleurial, and Jordan R. Chase of Caltech; Tim Knowles of Energy Science Laboratories; and Paul Morgan of Colorado Geological Survey for NASA’s Jet Propulsion Laboratory. NPO-49434
This Brief includes a Technical Support Package (TSP).
Venus Heat Flux Sensor
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Overview
The document outlines the development of a Venus Heat Flux Sensor by NASA's Jet Propulsion Laboratory (JPL), aimed at measuring heat flow on the harsh surface of Venus, characterized by extreme conditions of 460°C and 90 bars of pressure in a supercritical CO2 atmosphere. The sensor design features a flux plate, which consists of a thermal interface with the ground, layers of highly conducting and insulating materials, and an array of temperature sensors to measure temperature changes across the insulating layer.
Key challenges in measuring heat flow on Venus include operating at high temperatures, achieving good thermal contact on uneven surfaces, and obtaining precise measurements within the limited lifespan of passively cooled landers, which can only survive for several hours. The document discusses the surface conditions observed by the Venera Landers, noting that while some areas are rocky, most exhibit platy rock surfaces indicative of high-viscosity basalt flows. The research aims to assess the ability to measure thermal gradients in the presence of surface irregularities up to 1 cm in height.
To address the challenge of thermal contact, the team is developing a carbon fiber mat interface pad made from high-conductivity fibers. This pad is designed to maintain thermal conductivity even in the presence of surface irregularities. Testing has shown that thermal resistance increases significantly as the contact area decreases, but the design aims to ensure accurate gradient measurements despite this challenge.
The document also describes the development of a 1D analytic thermal model to estimate the necessary properties and dimensions of the flux plate to achieve a steady-state thermal gradient within two hours. Initial assessments suggest that using a low conductivity material, such as glass, is essential for measuring gradients across a thin plate.
Next steps in the research include testing more conformable carbon fiber pads, developing a numerical model for the flux plate, and constructing a breadboard prototype to demonstrate the measurement capabilities under Venus-like conditions. The research is supported by PIDDP funding and aims to enhance our understanding of Venus's thermal dynamics, contributing to broader planetary science.
