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thermal diffusivity
thermal diffusivity Measure of transfer of heat by diffusion
analogous to viscous motion; symbol α = λ/ρ C p .
thermal diode Solid-state generator of electricity
comprising layer of semiconductor at room temperature
joined by thermal insulative layer to layer heated to
250–450°C.
thermal efficiency Basic efficiency parameter of heat
engine, defined as percentage ratio of work done in given
time to mechanical equivalent of heat energy burned in
fuel supplied in same period. Usual symbol η T .
thermal emission EM radiation solely due to body’s
temperature (which if hot enough contains strong visible
radiation).
thermal excitation Acquisition of excess energy by
atoms or molecules as result of collisions.
thermal expansion Increase in dimensions caused by
increase in temperature.
thermal exposure Calories/cm 2 received by normal
surface in course of complete NW detonation (DoD).
thermal fatigue Mechanical fatigue caused by stresses
repeatedly imposed by thermal cycling.
thermal gradient See temperature gradient.
thermal gradiometer Airborne instrument for detecting
thermals by thermocouples on wing-tips which, in
presence of temperature difference, sends electrical signal
to cockpit indicator.
thermal heating Tautological; kinetic heating is meant.
thermal imagery Produced by measuring and electronically
recording thermal radiation from objects (NATO).
Normally IR wavelengths only are implied. Hence
thermal imaging, to produce pictorial displays or printouts
showing variation of temperature over field of view.
thermal index A forecast value of the temperature difference
between sinking and rising air.
thermal instability Any combination of temperature
gradient, thermal conductivity and viscosity resulting in
convective currents, eg wind in atmosphere.
thermal keel Generated by positioning engine jet nozzles
well forward under the fuselage [helps reduce generation
of sonic boom].
thermal lift 1 Lift due to thermal (1).
2 Lift imparted to air mass because of greater density of
cold surrounding air, not quite synonymous with (1).
thermal load Imprecise term usually meaning temperature
gradient or temperature stress.
thermally expanded metal Fabrication of parts from
aluminium alloy sheets rolled together with intervening
patterns of ‘ink’; the latter prevents the sheets bonding
and, on subsequent heating, expands to force the
unbonded parts to fit a mould.
thermal neutron Neutron slowed, eg in moderator, to
thermal equilibrium with surroundings at about 2,200 m/s
(so-called slow neutron); * analysis is principal method
used in detecting presence of explosives.
thermal noise RF noise caused by thermal agitation in
dissipative body (any conductor or semiconductor), also
called Johnson noise.
thermal paint Paint which changes colour very precisely
as the component is heated, afterwards remaining at the
colour appropriate to the highest temperature reached.
thermal picture synthesizer Matrix of heat-emitting thinfilm
resistors on Si substrate, each representing
individually addressed pixel to give overall large picture
rate of 50 Hz.
thermochemistry
thermal protection Protection against kinetic heating
during atmospheric entry (re-entry) of spacecraft structure,
RV or other body, esp. one intended for repeated
space missions.
thermal pulse Total IR emission from NW detonation,
or plot of IR flux against time during complete burst and
fireball climb.
thermal radiation 1 See thermal emission.
2 Total heat and light radiation produced by NW
detonation (DoD).
thermal relief valve Safety valve in fluid system to guard
against excessive pressure caused by overheating.
thermal runaway 1 Fault condition with element of
danger affecting Ni/Cd batteries characterized by particular
cells losing resistance (possibly because of high
temperature) and thus taking increased current, lowering
resistance still further in chain-reactive process.
2 Similar divergent overheating in current-carrying
transistor.
thermal sensitivity Of IR camera, quantified difference
in temperature required to output different tonal value
between black/white, typically 0.02–0.1°C.
thermal shock Severe mechanical stress resulting from
sudden extreme temperature gradient.
thermal soaring See soaring.
thermal stress See temperature stress.
thermal switch Switch activated by temperature difference
or particular temperature.
thermal thicket Flight conditions in which kinetic
heating (or other thermal problems) is a factor to be
considered but does not yet impose a thermal barrier
(colloq.).
thermal wind Notional vector difference between winds
at different heights, caused by horizontal variation of
atmospheric temperature and hence pressure at all upper
heights (note: not pressure surfaces).
thermal X-rays EM radiation, mainly in soft (low
energy) X-ray region, emitted by extremely hot NW
debris.
thermel Any device based on Seebeck, eg thermocouple,
thermopile.
thermie Non-SI unit of work (mechanical energy); 1 th
= 4.1855 MJ.
thermionic Involving electrons emitted from hot bodies.
thermionic converter Electric generator powered by hot
emitter and cold collector.
thermionic rectifier Depends on unidirectional electron
flow from cathode to anode.
thermionic tube See thermionic valve.
thermionic valve Evacuated capsule, usually glass,
containing heated cathode emitting electrons attracted to
anode, usually via one or more intervening control electrodes
usually called grids. In US called vacuum tube.
thermistor Protective resistor based on semiconductor
having high negative temperature coefficient of
resistance.
thermite Mixture of finely divided magnesium and iron
oxide used as heat source in welding and as incendiary
filling; originally spelt with capital T.
thermobaric warhead Creating both high-temperature
and blast-wave effects [often said of FAE].
thermobarograph Provides continuous readout of
temperature and pressure.
thermochemistry Branch of chemistry concerned with
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