Abstract
Determining current carrying capacity (ampacity) of wire bundles in aerospace vehicles is critical not only to safety but also to efficient design. Published standards provide guidance on determining wire bundle ampacity but offer little flexibility for configurations where wire bundles of mixed gauges and currents are employed with various external insulation jacket surface properties. Thermal modeling has been employed in an attempt to develop techniques to assist in ampacity determination for these complex configurations. An earlier tool allowed analysis of wire bundle configurations but was constrained to configurations comprised of less than 50 elements. Additionally, for vacuum analyses, configurations with very low emittance external jackets suffered from numerical instability in the solution. A new thermal modeler is presented allowing for larger configurations and is not constrained by low bundle jacket surface infrared emittance calculations. Formulation of key internal radiation and interface conductance parameters is discussed including the effects of temperature and ambient air pressure on wire-to-wire thermal conductance. Test cases comparing model-predicted ampacity and that calculated from standards documents are presented.
Highlights
Ampacity, a term for amperage capacity, is a measure of the current carrying capability of a wire or a collection of wires in a bundle
Use of the standards is limited as none of the publicly-available standards provide a procedure to determine the unmargined maximum current on a conductor within a bundle, nor do they allow for mixed wire sizes, different wire jacket emittances, or a variety of currents on individual conductors
For a single wire under steady state conditions, the resulting conductor temperature is readily calculated by establishing a heat balance, i.e., the rate at which is heat generated within the wire due to ohmic heating must be equal to the rate at which it is rejected from the wire
Summary
A term for amperage capacity, is a measure of the current carrying capability of a wire or a collection of wires in a bundle. Heat transfer between adjacent wires can occur due to direct contact between adjacent insulation jackets, via radiation, and via air conduction in the gaps between wires for cases where an atmosphere is present. First order radiation heat transfer was considered (i.e., direct radiation heat transfer between adjacent wire jackets with no reflection off of other wire jackets in view of the two wires of interest); 2 Both wire jackets were assumed to have the same infrared emittance; 3. Under these assumptions, Grad values were calculated for the range of possible wire size combinations (using the Cullimore and Ring Technologies, Inc. RadCAD® application) by modeling adjacent wire jackets as infinite cylinders and normalizing the results based on radius ratios (Figure 2)
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