The Heat Transfer From Electrically Heated Wire Wound Coils and the Ignition of Methane/Air Mixtures by Such Coils

The flow characteristics of a gas passing a heated coil determine the heat transfer characteristics and, for a combustible mixture, also the ignition characteristics. This study describes and gives explanations for the heat transfer Characteristics and the ignition characteristics of a heated wire wound coil, noncatalytic.
In the heat transfer study the coil parameters range from Wire diameters of 0.279mm to 0.56mm, coil diameters or 2mm to 7mm, coil pitches of 0.47mm to 3.4mm and number of coil turns from 3 to 15. Flow parameters range from free stream velocities of zero to 400mm seec-1 with coil surface temperatures of 800 to 1100°c. The maximum Reynolds no. based on coil diameter is 37.
The heat transfer results over the above range of Parameters are correlated Within ± 20% by the following expression :-
ÉG = 0.2 + 0.41 P – 0.06P2
Where Eg is the dimensionless: heat transfer defined as EG/EG,MAX and P is a dimensionless pitch defined as P2/ndo-do2; do is the wire diameter
Using a simplified mathematical model of a coil expressions for the radiation emitted by a close wound coil have been calculated for the first time. The results are in good agreement With those found in practice and the expressions are shown below.
[see page 4]
The radiation absorbed in the above equations is expressed aa a fraction of the total radiation emitted by a ring of square cross-section Where D is the ring diameter, equating to the coil diameter, rc is the radius of the ring, equating to the coil radius, r, is half the length of the cross-section side
of the ring equating to the wire radius, and X2 is the separation of the absorbing faces. In addition a= x22 + 2rc2 and b=-2rc2.
Flow visualisation techniques have been extensively used. For the first time an understanding of the flow around a hot coil when individual turns are brought into close proximity and interaction between the flow around each turn occurs has been gained. This understanding has been assisted by a study of the interaction between the flows, under natural convection conditions around two and three heated rods.
Using very simple considerations based upon formulae for the thermal boundary layer thickness the optimum separation of the rods and of the coil turns for heat transfer is predicted.
The ignition characteristics are examined over the following range oi coil and flow parameters:
Wire diameters from 0.279 to 0.56mm; coil diameters from 2 to 7mm; coil pitches from 0.47 to 2.0mm; no. of Coil turns from 3 to 15; free stream velocities from 50mm sec-1 to 400mm sec-1 and mixture compositions from 3 to 1 3% CH4 in air.
A qualitative explanation of the ignition characteristics is given for the first time and in particular an explanation given for the existence of an optimum pitch for ignition. This explanation is based on the overlap of thermal boundary layers as the coil turns are brought into close proximity and upon a postulated quenching effect inside the coil.
High speed shadow and schlieren photography of the ignition sequence has played a major part in the understanding of the ignition characteristics.
The optimum design of an igniter coil is presented.