thermal dimpling. See hot dimpling.
thermal efficiency (heat engine specification). The ratio of the amount of useful work produced by a heat engine to the amount of work that could be done by the heat energy in the fuel used.
thermal expansion. The change in the physical size of a material caused by the material absorbing heat. Thermal expansion causes a material to expand both in its length and in its volume.
thermal expansion coefficient. A dimensionless number that relates to the change in the physical dimensions of a material as the temperature of the material changes. The thermal expansion coefficient of aluminum is approximately twice that of steel.
thermally actuated switch (electrical component). A switch whose contacts are actuated by changing temperature. The contacts are normally mounted on a bimetallic strip that warps when its temperature changes. When the strip warps, the contacts either snap closed or snap open, depending on whether the switch is actuated by a descending or a rising temperature.

thermal relief valve (hydraulic system component). A pressure relief valve in a hydraulic system that relieves the pressure which builds up when heat expands the fluid. Thermal relief valves are installed in sections of the system in which fluid can be trapped, usually in the lines between the selector valves and the actuators. When fluid in these lines absorbs heat and expands, the thermal relief valve momentarily unseats and releases enough fluid back into the system return manifold to relieve the high pressure and prevent damage to the system.
thermal runaway (nickel-cadmium battery malfunction). A condition that can occur in a nickel-cadmium battery in which the battery overheats because of excess current. A nickel-cadmium battery has a very low internal resistance, but if an excessive amount of current flows from it, even its low resistance will cause it to overheat. The center cells get hotter than the outer cells, which transfer some of their heat to the outside air, and when the center cells overheat, their voltage and resistance both drop.