Sánchez Tarifa, Carlos and Liñán Martínez, Amable and Salvá Monfort, José Juan and López Juste, Gregorio and Tizón Pulido, Juan Manuel and Cura Velayos, Juan Manuel del
Study on combustion processes in reduced gravity : final report volume I.
Monografía (Project Report).
E.T.S.I. Aeronáuticos (UPM).
1. ABSTRACT OF THE WORK
1.1. GENERAL CONSIDERATION ON THE WORKING PACKAGES
The following tasks have been carried out under the present Contract:
WP1. Continuation of the experiments on flame spreading in parabolic aircraft flights, conducted in one flight campaign. These experiments have been, in part, a continuation of the preceeding research programmes and they were directed to the study of the influence of fuel thickness at reduced gravity on the flame spread velocity.
In addition, a few experiments were carried out in order to obtain some preliminary information on possible combustion experiments to be performed in a sounding rocket module.
WP2 and WP3. These works on the sounding rocket module and on the sounding rocket experiments were preceeded by a common study, which consisted in a review of the combustion experiments that could be carried out in a sounding rocket module. The requirements of the experiments were analised, with special emphasis on the essential factors of the time and space needed for each possible experiment.
It was definitively concluded that there is no practical way to keep constant the gaseous atmospheric composition in the module chamber throughout a stationary combustion process in a still atmosphere. This is due to the fact that there is not any practical feasible way to extract the combustion products and to feed into the module the oxygen
or the reactant gases consumed without strongly disturbing the flow field. This occurs because under microgravity conditions gas velocities in combustion processes at constant pressure are essentially controlled by diffusion and they are usually very small.
3. As a consequence, the module has to contain a sufficient amount of oxidizer such that its variation during the combustion process should be permissible.
This requirement imposes limitations in the volume and combustion time, which are interdependent and in the size of the experiment. In addition, the volume available has to be sufficiently large in order to avoid significant interference of the walls of the module on the combustion field. If the experiment is of a non-stationary nature
and the flame size increases with time, as it occurs in many
combustion processes, this imposes another limitation in the
combustion time and in the size of the experiment.
The aforementioned review of the combustion processes is shown in this Final Report and from its conclusions the basic data for the study and specifications of the combustion module were obtained.
1.2. MULTIPLE EXPERIMENTS
From the aforementioned review it was concluded than in most
combustion experiments times of the order of one minute would be sufficient. Since there are six minutes at reduced gravity available, the problem of utilizing the module for several experiments has to be considered, specially taking into account the high cost of the launching of a sounding rockets.
Except for the experiments in which a little amount of oxidizer is consumed, such as in droplet combustion, reutilization of the module for multiple experiments require emptying the module and refilling it with
the specified gaseous mixture. This process is mechanically simple but it presents the difficult problem of knowing the time required for the oxygen or gaseous mixture introduced into the chamber to become at rest. The process is asymptotic, therefore it has to be specified the minimum value admitted for the gas fluctuation velocities (for example <2 mm/s). The analytical solution of this problem is very difficult, and it is presently being studied by NASA. Considering its importance, the problem has been studied experimentally under the present Contract. It has been concluded after a series of experiments with different tracers, that after elapsing times of the order of two minutes the fluctuation velocities were lower than 2 mm/s, which were considered sufficiently small. This implies that two or three refills are possible in each flight.
A large amount of experiments would occupy the full available diameter of the module [-40 cm). Some experiments would need smaller diameters. This opens the possibility of utilizing several chambers inside the module, increasing the possibility of multiple experiments for each flight and reducing costs, and avoiding refilling in some cases.
A preliminary desing of this multiple combustion chamber solution has been carried out.
1.3. FORCED CONVECTION EFFECTS AT LOW REYNOLDS NUMBER
Forced Convection effects are currently considered of high interest in combustion processes at reduced gravity. This is due to two reasons: in the first place, forced convection effects at low Reynolds numbers cannot be experimentally studied at one g on the ground, because they
are masked by free convection effects. On the other hand, if a fire takes place in a spacecraft or space laboratory it will be most likely take place in an environment of low velocity gas currents originated by the air conditioning equipment or even by the motion of people.
Forced convection effects can be studied in the sounding rocket module in two different ways: by moving the burning smaple along the module keeping the oxygen or oxidizers at rest, or by keeping the sample at rest and producing a continuous flow of gas at a given velocity.
Although their study was not specified in this Contract, both systems have been analysed. The first system is relatively simple, requiring refilling of the module. Three or four experiments would be possible to carry out in each
flight. The second systems is mechanically very complicated requiring two valves controlled probably by a microcomputer. A specific system is proposed and studied. Its principal controlling laws are discussed, as well as the main problems and possible solutions. In principle, this system offers the possibility of carrying out more experiments per flight than the first system. However, some
important transient problems have to be thoroughly studied, requiring a test programme before concluding that the proposed system is a viable or better solution.
1.4. WP4. THEORETICAL STUDIES
The theoretical studied have been devoted to quiescent combustion and flame spreading utilizing numerical methods and an order of magnitude analysis. Several important results have been obtained, as for example the influence of gravity on flame spreading depending on the fuel thickness,
which have been experimentally verified during the parabolic flights. In addition a study on the influence of forced convection on the flame spreading process has been carried out.