GB775365A - Improvements in or relating to combustion apparatus - Google Patents

Abstract

775,365. Generating combustion products under pressure. NATIONAL RESEARCH DEVELOPMENT CORPORATION. April 21, 1955 [May 6, 1954], No. 13213/54. Class 51 (1). In combustion apparatus for burning fuel in a high-velocity stream of combustion-supporting gas, a combustion zone is provided and is so designed as to pass gas there-through at a predetermined optimum volumetric rate of flow per unit volume of the combustion zone and designed for operation at a combustion zone pressure greater than a critical pressure below which combustion becomes unstable. The critical pressure for any one mixture strength is related to the aforesaid pre-determined optimum volumetric rate of flow in such a manner that appreciable deviation therefrom is accompanied by an increase in the critical pressure. Curves plotted on a log-log basis showing the relation between critical pressure in a combustion zone, and values of volumetric rate of flow per unit volume of the combustion zone (i.e. Ua/v where " U " is the average normal velocity of gas entering the combustion zone, " a " is the cross-sectional area of the zone and " v " is the volume of the combustion zone) are shown in Fig. 1 for propane-air mixtures of various airfuel ratios, the lowest curve being for an air-fuel mixture of the stoichiometric ratio. Combustion apparatus according to the present invention may be divided into two combustion zones, one operating at a high value of the parameter Ua/as shown, for example, by the dotted line A, v, while the other operates at a value of the parameter Ua/v substantially equal to the optimum value as shown by the dotted line B. The dotted line A is shown for the value of Ua/v equal to 20 but the value may vary between 20 and 200 sec.<SP>-1</SP>, while the dotted line B is shown for the value equal to 2 which is the approximate optimum value for propane/air mixtures. One section of the combustion apparatus operates at the cruising condition for values of Ua/v between 20 and 200 while another section operates at the idling condition for values of Ua/v approximately equal to 2. In one arrangement of a gas turbine engine combustion equipment, Fig. 2, a number of interconnected flame tubes 10 each constituting an idling combustion zone, and a number of interconnected flame tubes 11 each constituting a cruising combustion zone, are provided. In a second embodiment, Fig. 4, two annular flame tubes 16, 17 are disposed within an annular casing 13, the flame tubes 16, 17 constituting the cruising and idling combustion zones respectively, the cruising zone being smaller in volume than the idling zone. In another embodiment only one flame tube is provided, the upstream section thereof constituting an idling combustion zone and the downstream section constituting the cruising combustion zone. In another embodiment, Fig. 6, a single combustion chamber 38 is employed to constitute both an idling and a cruising combustion zone, the volumetric flow of air through the combustion chamber being varied by an adjustable ring valve 41. The ring valve 41 is opened at idling conditions so that only a small proportion of air delivered by the compressor 35 flows through the combustion zone, the volumetric flow through the combustion chamber being at the optimum value of the parameter Ua/v. At cruising conditions, the ring valve 41 is closed or partly closed whereby the volumetric flow through the chamber is increased to the " cruising " value of the parameter Ua/v. In a further embodiment, the invention is shown applied to combustion apparatus as disclosed in Specification 758,345, in which a series of deflector plates are disposed at a region of sudden enlargement in a duct through which combustion-supporting gas is flowing. The space between one pair of deflec. tor plates operates as an idling and pilot combustion zone, while the space between a second pair of plates operates as a cruising combustion zone.