Thermal FAQ, Volume 1

Volume 1. A B C D E F G H I J K L M
Volume 2. N O P Q R S T U V W X Y Z

PCAnalyze TAK 2000 Thermal Connection


Absolute Zero:
Temperature at which thermal energy is at a minimum. Defined as 0 Kelvin, calculated to be -273.15°C or -459.67°F.

Albedo:
Earth reflected solar radiation.
Perihelion Aphelion Mean
0.30+/-0.01 0.30+/-0.01 0.30+/-0.01
global annual average

Ambient Temperature:
The average or mean temperature of the surrounding air which comes in contact with the equipment and instruments under test.

Analogous Systems:
Two systems are said to be analogous when they both have similar equations and boundary conditions and the equations can be transformed into the equations for the other system by simply changing symbols of the variables. Thermal and electrical systems are two such analogous systems.
Quantity Thermal System Electrical System
Potential T E
Flow q I
Resistance R R
Conductance G 1/R
Capacitance C C
Ohm's Law q=GT I=E/R
The analogy between thermal and electrical systems allows the engineer to utilize the widely known basic laws such as Ohm's Law and Kirchhoff's Laws used for balancing networks. Numerical techniques such as finite differencing, are used to solve the partial differential equations describing such systems. TAK 2000 has been developed around such techniques. It enables engineers to readily compute temperature distributions and gradients of very complex thermal networks.

Arithmetic Nodes
An arithmetic node can be used to represent the surface of a material. It could also represent the interface between two dissimilar materials, (for example a bondline). Arithmetic nodes have no thermal capacitance. They are sometimes called steady state nodes. Their temperatures are calculated by being brought into a steady state heat balance with the neighboring nodes. It can be used to represent nodes with very small capacitance relative to the rest of the model. In a transient analysis, this could result in a significant reduction in computer run time with only minor changes in overall accuracy.

Blackbody:
A theoretical object that radiates the maximum amount of energy at a given temperature, and absorbs all the energy incident upon it. A blackbody is not necessarily black. (The name blackbody was chosen because the color black is defined as the total absorption of light energy.) For example freshly fallen snow and white paint have an IR absorptivity approaching 0.95

BTU:
British thermal units. The quantity of thermal energy required to raise one pound of water at its maximum density, 1 degree F. One BTU is equivalent to .293 watt hours, or 252 calories. One kilowatt hour is equivalent to 3412 BTU.

Boundary Nodes:
Boundary nodes are used to represent constant temperature sources or sinks. Effectively, they have infinite thermal capacitance. Boundary conditions such as ambient air, electronic baseplates, or deep space can be simulated by using boundary nodes.

Boundary node temperatures are not altered by the solution routines. However, time varying boundary conditions can be modeled with modern thermal analyzers.

Calorie:
The quantity of thermal energy required to raise one gram of water 1°C at 15°C.

Capacitance (thermal):
A thermal modeling term. The capacitance C of a node is computed from the thermophysical properties of the subvolume evaluated at temperature T of the node.

C = M * Cp

where:

C Capacitance of the node
M Mass of the node
Cp Specific Heat of the node

Steady state thermal modeling solutions are not dependent upon thermal mass The transient solution routine does require the thermal mass of the nodes. Nodes with small thermal capacitance (when compared to the rest of the model) can be input as arithmetic nodes. The computational time step used in the transient solution is driven by small thermal capacitance diffusion nodes which are connected by large thermal conductors. Therefore, arithmetic nodes, when used with discretion, can save considerable computer time.

Celsius (centigrade):
A temperature scale defined by 0°C at the ice point and 100°C at boiling point of water at sea level.

Conductance:
The measure of the ability to carry a heat flow.

Conduction (thermal):
A thermal modeling term. Heat flows from a region of higher temperature to a region of lower temperature. Conduction is the process by which heat flows within a medium or between different mediums in direct contact. The energy is transmitted by molecular communication.

Conductors which represent conduction or convection paths are referred to as linear conductors because the heat flow is a function of the temperature difference between nodal temperatures to the first power.

Qdot = G * (T1 - T2)

Linear conductors representing solid conduction are computed from the equation:

G = k * A / L

where:

G thermal conductance (i.e. Btu/hr-F or W/C )
k thermal conductivity (i.e. Btu/hr-ft-F or W/cm-C )
A cross-sectional area through which heat flows (i.e. FT2 or cm2 )
L length between adjoining node centers ( i.e. ft or cm )

Conductivity (thermal):
The property of a material to conduct heat in the form of thermal energy. Typical values.

Conductors:
A thermal modeling term. Conductors are used to represent heat flow paths from node to node. Two basic types of conductors may be defined by the modeler: linear and radiation. In this network, conductor "G" connect nodes "T1" and "T2" which have a thermal capacitance of "C1" and C2".

Convection:
1. The circulatory motion that occurs in a fluid at a non-uniform temperature owing to the variation of its density and the action of gravity. 2. The transfer of heat by this automatic circulation of fluid.

For heat transfer by convection, the conductor is calculated by the following equation:

G = hc * A

where:

hc is the convection coefficient (energy/length2-time-deg)
A surface area in contact with the fluid (length2)

Cryogenics:
Measurement of temperature at extremely low values, i.e., below -200°C.

Density:
Mass per unit of volume of a substance. I.E.: grams/cm3 or pounds/ft3

Diffusion Nodes:
A diffusion node is used to represent normal materials. Diffusion nodes have thermal mass (capacitance) and store and release energy with time. This process is characterized by a gain or loss of potential energy which depends on the capacitance value, the net heat flow, and the time over which the heat is flowing. In the transient solution routine, diffusion node temperatures are calculated by a finite difference representation of the partial differential heat transfer equation. Typically three items are stored for each diffusion node: temperature, thermal capacitance, and nodal heating (if any).

Thermal capacitance is the product of the mass of the node and the specific heat of the material that comprises the node. The mass can be calculated and the Specific Heat can be found in reference materials.

Earth radius:
6.37E+06 meters

Emissivity:
The ratio of energy emitted by an object to the energy emi tted by a blackbody at the same temperature. The emissivity of an object depends upon its material and surface texture; Typical values. Yet more values! Checkout this site for the last word om emmissivity.

Endothermic:
Absorbs heat. A process is said to be endothermic when it absorbs heat.

Enthalpy:
The sum of the internal energy of a body and the product of its volume multiplied by the pressure.

Eutectic Temperature:
The lowest possible melting point of a mixture of alloys.

Exothermic:
Gives off heat. A process is said to be exothermic when it releases heat.

First Surface Mirrors:
First surface mirrors usually have polyimide or polyester substrates. Multi-layer insulation blankets use first surface mirrors for infrared head reflection.

The most common metal used is aluminum, followed by gold and on rare occasion, silver. All metals have low emittances.

Aluminized polyester curbs insulation blanket costs for large surfaces. Higher temperature requirements or non-burning material requirements dictate a polyimide substrate.

Metal
Deposit
Typical
Emittance
Typical
Absorptance
Gold 0.02 .28
Silver 0.02 0.07
Aluminum 0.03 0.12

Freezing Point:
The temperature at which the substance goes from the liquid phase to the solid phase.

Heat Sink:
1. Thermodynamic. A body which can absorb thermal energy. 2. Practical. A finned piece of metal used to dissipate the heat of solid state components mounted on it.

Heat Transfer:
The process of thermal energy flowing from a body of high energy to a body of low energy. Means of transfer are:
  • Conduction
  • Convection
  • Radiation
  • Mass Flow

Heat:
Thermal energy. Heat is expressed in units of calories or BTU's. In the real world, there are many reasons why thermal energy can enter a system. For example;
  • Electrical components produce Joule heating.
  • Two parts rubbing together can generate frictional heat
  • A clothes dryer with a gas burner
  • Electric elements of a toaster
  • A laser beam striking a mirror will leave a part of its energy with the mirror.
  • Gamma radiation can interact with the atomic structure of a material to cause internal heating.
  • A satellite, orbiting the earth, is exposed to three major types of environmental heating.
    • Direct sunlight incident on its surfaces
    • Albedo, or reflected sunlight, from the surface of the planet
    • Infrared (IR) radiation from the planet.

Heat Pipe:
A heat pipe is a self contained heat pump that has the capability of transporting heat at a high rate over fairly substantial distances with no external pumping power. The heat pipe is composed of three sections; an evaporator, an adiabatic section, and a condenser. A porous capillary wick covers the inside surface and extends over the length of the pipe. Heat is applied at the evaporator end and is removed at the condenser end. At the evaporator end, heat applied by the external source vaporizes that portion of the working fluid located at the evaporator. This creates a difference in pressure between the evaporator and the condenser ends which drives the vaporized fluid through the adiabatic section. At the condenser end the heat is removed by condensation.

Heaters:
A proportional heater controller varies continuously (and smoothly) as a function of the temperature of a control sensor. A thermostatic heater is turned full on or off as a function of the temperature of a control sensor.

Infrared:
An area in the electromagnetic spectrum extending beyond red light from 760 nanometers to 1000 microns (106 nm). It is the form of radiation used for making non-contact temperature measurements.

Insulation Resistance:
The resistance measured between two insulated points on a transducer when a specific dc voltage is applied at room temperature.

Isothermal:
A process or area that is a constant temperature.

Joule:
The basic unit of thermal energy.

Kelvin:
Symbol K. The unit of absolute or thermodynamic temperature scale based upon the Celsius scale with 100 units between the ice point and boiling point of water. 0°C = 273.15K

Latent Heat:
Expressed in BTU per pound. The amount of heat needed (absorbed) to convert a pound of boiling water to a pound of steam.

Linear Conductors:
A thermal modeling term. The conductance of a linear conductor is input in units of energy per unit time per unit degree, and the heat rate through such a conductor is calculated in the network solution routines as:

Q = G * (Ti - Tj)

where:

Q Heat rate (energy/time)
G Conductance (note G = 1 / R )
T Temperature

Several types of physical heat transfer mechanisms can be modeled as linear conductors, including conduction, convection, and mass flow.

Louvers:
Louvers are active thermal control devices used on spacecraft. They are devices which mask a high-emissivity radiator with a low-emissivity surface. As the temperature of the radiator gets hotter, the mask is withdrawn, exposing the radiator. The drive mechanism is usually a bi-metallic actuator, but active control could be used. There are several styles of louvers. A bladed version is shown below.

Mass Flow Conductor:
A thermal modeling term. Mass flow conductors are actually a special type of linear conductor. The use of a mass flow conductor in a thermal network is a convenient method for the transfer of energy from one point to another due to the actual movement (flow) of a fluid from one point to another. The mass flow conductor simply accounts for the internal energy term of a mass moving from one location to another. Such a conductor is defined by prefixing the up stream node number with a minus sign. The node so designated will not be allowed to lose or gain heat through the conductor, even though its temperature will be used to calculate a heat flow to the downstream node. Mass flow conductors are computed from the equation:

G = Mdot * Cp

where;

G the flow conductor (energy/time-deg)
Mdot is the mass flow rate (mass/time)
Cp is the specific heat of the fluid (energy/mass-deg)

Melting Point:
The temperature at which a substance transforms from a solid phase to a liquid phase.

Mil:
One thousandth of an inch (.001").

Millimeter:
One thousandth of a meter, symbol mm.
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