COMBUSTION OF STRATIFIED HYDROGEN-AIR MIXTURES IN THE 10.7 M(3) COMBUSTION TEST FACILITY CYLINDER

This paper presents preliminary results from hydrogen concentration gr adient combustion experiments in a 10.7 m(3) cylinder. These gradients , also referred to as stratified mixtures, were formed from dry mixtur es of hydrogen and air at atmospheric temperature. Combustion pressure s, burn fractions and flame speeds in concentration gradients were com pared with combustion of well-mixed gases containing equivalent amount s of hydrogen. The studied variables included the quantity of hydrogen in the vessel, the steepness of the concentration gradient, the ignit er location, and the initial concentration of hydrogen at the bottom o f the vessel. Gradients of hydrogen and air with average concentration s of hydrogen below the downward propagation limit produced significan tly greater combustion pressures when ignited at the top of the vessel than well-mixed gases with the same quantity of hydrogen. This was th e result of considerably higher burn fractions in the gradients than i n the well-mixed gas tests, Above the downward propagation limit, grad ients of hydrogen ignited at the top of the vessel produced nearly the same combustion pressures as under well-mixed conditions; both gradie nts and well-mixed gases had high burn fractions. Much higher flame sp eeds were observed in the gradients than the well-mixed gases. Gradien ts and well-mixed gases containing up to 14%, hydrogen ignited at the bottom of the vessel produced nearly the same combustion pressures. Ab ove 14% hydrogen, gradients produced lower combustion pressures than w ell-mixed gases having the same quantity of hydrogen. This can be attr ibuted to lower burn fractions of fuel from the gradients compared wit h well-mixed gases with similar quantities of hydrogen. When ignited a t the bottom of the vessel, 90% of a gradient's gases remained unburne d until several seconds after ignition. The remaining gases were then consumed at a very fast rate.