RU2116093C1 - Device for determination of material burning range by flow velocity for zero-gravity state - Google Patents

Abstract

FIELD: determination of parameters of fire hazard of structural materials under conditions of orbital flight, including ranges of their burning by flow velocity in zero gravity state. SUBSTANCE: device has combustion chamber in the form of flat channel, mixer of nitrogen and oxygen, gas analyzer, material specimen holder and drive for moving the holder. The invention is based on the fact that with reduced thickness of flat gas layer confined between horizontal plates, naturally convection is practically fully discontinued and the process of heat-mass-transfer in firing takes place only due to forced convection as in zero-gravity state. EFFECT: higher efficiency. 2 dwg

Description

 The device is designed to determine a baseline characterizing the fire hazard of materials in zero gravity conditions, which is the lower limit of materials burning at a flow rate in zero gravity. This indicator is necessary in the development of fire safety means for inhabited pressurized compartments of spacecraft (SC), in particular long-term orbital stations and interplanetary spacecraft.

 Inhabited pressurized vessels of the spacecraft belong to objects of increased fire hazard, which is due to the increased concentration of oxygen in the atmosphere of the pressurized compartments; the use of a large number of polymeric materials, which is determined by the requirement to reduce the mass of the equipment; high saturation of pressurized compartments with electrical equipment, the elements of which in an oxygen-enriched environment especially often become sources of fire; the inability to assist the crew from outside.

 The solution to the issue of ensuring fire safety of habitable pressurized compartments of spacecraft with traditional methods encounters serious technical difficulties. First of all, this is due to the need to meet the conflicting requirements that are imposed on the means of ensuring fire safety of the inhabited pressurized spacecraft, on the one hand, the pressurized compartments are assigned the ultimate responsible functions in the space-rocket system and the means of ensuring fire safety must be reliable and fast-acting, and on the other - the use of traditional means and methods of ensuring fire safety, for example, fire extinguishing means using fire extinguishers ETS, is constrained by the strict limitations on mass equipment installed on board and ecology habitable environment.

Investigations of the processes of ignition and combustion of materials in zero gravity made it possible to identify limiting phenomena during the combustion of solid materials, which could not be detected in the presence of gravity due to the determining effect of natural convection on combustion under these conditions. It was established that there is a lower limit for the combustion of materials by the flow rate in zero gravity (V _CR ), i.e. the value of the flow rate below which the combustion of this material in zero gravity does not occur.

 The results of the studies made it possible to develop fundamentally new ways to ensure the fire safety of the inhabited pressurized spacecraft in terms of both preventing fire and extinguishing it.

Thus, to implement new approaches to ensuring fire safety in the pressurized compartment of the spacecraft, it is necessary for materials intended for use in the pressurized spacecraft to determine the values of V _pr taking into account the influence of atmospheric parameters, oxygen concentration in the atmosphere (C _ok ) and its pressure (P _cf ) .

Known devices for determining the flammability of materials with different oxygen concentrations in the atmosphere and its pressure, for example, a device for determining the limit of combustion of materials on the oxygen concentration (C _etc.) at different pressures (IA Bolodyan Dolgov EI, Zhevlakov AF ., Melikhov A.S. et al. Explosion Combustion Physics., 1979, pp. 63-65 N 4).

The device includes a vertically mounted cylindrical combustion chamber. From below, a gas mixture with a predetermined oxygen concentration (C _approx ) is supplied from the mixer with a certain flow rate to the combustion chamber. For a uniform distribution of the gas mixture over the cross section of the combustion chamber, a porous element (mesh packet) is installed in its lower part. Using the holder, the test material sample is set along the axis of symmetry of the combustion chamber. To determine the combustion limit of the material by the concentration of oxygen in the combustion chamber, a gas stream is created with a certain initial oxygen concentration. If, in this case, a stable burning of the material sample is observed, then in the next experiment the oxygen concentration in the mixture is reduced. Thus, the limiting concentration of oxygen is found below which the combustion of the material is impossible. This value is used as an indicator of the combustibility of the material in an atmosphere enriched with oxygen.

 The disadvantage is that it is impossible to determine the fire hazard indicators of materials for zero gravity conditions, because The combustion of materials in the combustion chamber is ensured mainly by natural convection, and the forced gas flow acts as a ventilating chamber from the combustion products. The closest analogue is a device that includes two containers: internal and external. The inner container, in which the burning sample of material is placed, is located at the top of the outer container before dropping. In the process of free fall, the outer container takes on the aerodynamic drag of the air, which ensures a high degree of weightlessness in the inner container. The sample is placed in a combustion chamber with a rectangular cross section along its axis and can be blown by a gas stream with specified parameters (gas mixture flow rate and oxygen concentration in it). The camera is made with transparent walls, which provides the possibility of filming a movie camera installed in an internal container. (Melikhov A.S., Potyakin V.I., Ryzhov A.M., Ivanov B.A. On the limit regimes of polymer combustion in the absence of free convection. Explosion Combustion Physics., 1983, N 4, p. 27-30) .

 In the channel with the sample installed in it, a gas flow was generated at a given speed. The sample was ignited by an electric spiral, which was removed outside the channel and the containers were freed from suspensions. For some time, combustion proceeded in zero gravity. At the same time, filming of the combustion process of a sample of material was carried out. If stable combustion of the sample was observed at a given flow rate, then in the next experiment the flow rate decreased. Thus, the limiting velocity of the gas flow was determined below which the combustion of the material in zero gravity was not maintained.

Determination of V _{ol by} these means is a laborious and expensive work. The complexity of the experiment is due to the short duration of the state of zero gravity on these funds. With their help, V _pr can be determined for a narrow range of materials with a short thermal relaxation time of the combustion zone for, for example, thermoplastics. For testing materials that form a coke residue during combustion, the method for determining V _ol with a short period of zero gravity was practically unsuitable.

 An object of the invention is to develop a device that would allow to determine important data on the ignition and combustion of materials for orbital flight, without resorting to creating a state of zero gravity, i.e. under the action of gravity.

где
P=P₀/P_ср - член, характеризующий давление газовой среды;
C_ок - объемная доля кислорода в газовой среде;
g - ускорение силы тяжести, см/с²;
K_g - ампирический коэффициент;
P₀ - атмосферное давление, МПа;
P_ср - давление газовой среды в опыте, МПа.The problem is solved in that one of the walls of the chamber, installed in a horizontal position, is movable, while the height of the combustion chamber when determining the combustion limit of materials for weightlessness, depending on the parameters of the working gas atmosphere in which the material is supposed to be used, is established based on the ratio

Where
P = P ₀ / P _cf - the term characterizing the pressure of the gaseous medium;
C _ok - volume fraction of oxygen in the gas medium;
g is the acceleration of gravity, cm / s ² ;
K _g is the empirical coefficient;
P ₀ - atmospheric pressure, MPa;
P _cf - gas pressure in the experiment, MPa.

In FIG. 1 is a diagram of a device for determining the combustion limit of materials by flow rate, FIG. 2 is a comparison of the dependences of the value V _pr of the channel heights h _k obtained for f organic glass (PMMA).

The device comprises a combustion chamber 1, made in the form of a flat channel for organizing a uniform flow cross-section through its cross section, a grid of 2 is installed. The chamber is made of two horizontal massive plates, the use of which excluded their local heating. Structurally, using a movable upper plate, a change in the distance between the plates h _k was provided. The sample material 3 is placed on the rod, which can be moved using the actuator 4, holding the frontal part of the sample on the axis of the channel. A gas stream with a given oxygen content was prepared in mixer 5. The gas flow rate and oxygen concentration in it were measured using a flow meter 6 (PC-3) and a gas analyzer 7 (zircon-M).

To determine the values of V _pr materials at different pressures, the device was placed in a sealed vessel.

 To exclude the influence of natural convection on the results of the experiments, the combustion chamber 1 was installed strictly horizontally.

 The design of the drive 4 ensured the installation of the end face of the sample of material that was ignited before being introduced into the channel at a given distance δ from the grids 2.

 To set the experimentally necessary flow rate flowing to the combustion zone, special calibration experiments were carried out in which the flow rate was determined by visualizing the gaseous medium with smoke - the speed of movement in the channel of a small (about 5 mm) smoke cloud based on a given length in location of the intended location of the hot portion of the sample.

The experimental procedure was as follows:
- the ability of this material to burn at a given oxygen concentration was determined (C _pr value), V _pr value for the material was determined if C _pr turned out to be less than C _ok ;
- depending on the parameters of the gaseous medium in the experiment (C _ok and P _cf ), the distance h _k to between the plates was established by the formula (1);
- a sample of material was installed in the holder (samples of monolithic materials were made in the form of a plate 1 x 8 x 60 mm, samples of fabrics and films 30 x 60 mm in size were installed in a flat holder made in the form of a fork with an open leading edge, all types of samples were installed horizontally on equal distances from the horizontal walls of the chamber);
- in the chamber using a mixer, a stream of nitrogen-oxygen mixture was created with a given speed and oxygen concentration;
- the sample was ignited outside the chamber in a stream of a gaseous medium emerging from it and after the start of stable combustion for a period of at least 3 s, it was introduced into the chamber so that the burning frontal part was located at a distance of 10 mm from the grids;
- combustion was monitored, the preservation of the shape of the sample and the placement of the combustion zone in the medium-high plane of the chamber were recorded;
- for materials, during the combustion of which there was no coke residue or solid skeleton, the flow velocity at which combustion continued for at least 20 s was taken as the value of V _pr ;
- for materials, during the combustion of which a solid skeleton remained, the flow rate at which burning continued for at least 10 s was taken as V _pr .

где
Р=Р_с/Р_ср - член, характеризующий давление газовой среды;
С_ок - объемная доля кислорода в газовой среде;
g - ускорение силы тяжести, см/с²;
K_g - эмпирический коэффициент;
P₀ - атмосферное давление, МПа;
Р_ср - давление газовой среды в опыте, МПа.The main parameter of the device, the knowledge of which ensures the determination of V _ol in the presence of gravity, is h _to . As a result of processing the data obtained at different channel heights h _k , C _ok and P _cf (example in Fig. 1), a formula was compiled to determine the value of h _k k depending on C _ok and P _cf ;

Where
P = P _c / P _cf - the term characterizing the pressure of the gaseous medium;
With _ok - the volume fraction of oxygen in the gas environment;
g is the acceleration of gravity, cm / s ² ;
K _g is the empirical coefficient;
P ₀ - atmospheric pressure, MPa;
P _cf - the pressure of the gas medium in the experiment, MPa.

The comparison of the results of determining the values of V _CR obtained for several materials using the inventive device, i.e. in the presence of gravity, and in a free-fall container, i.e. in true zero gravity. In the table, on the example of dry glue VK-11, it is shown that the proposed device allows with a small error (not more than 15% for different materials) to obtain V _pr values for materials on Earth that are adequate to those measured in zero gravity.

Verification of the objective positions has been made, incorporated in a device for determining V _pr. With shlirenmetoda (shadow filming) has been found that when working values of _k h and certain values of V _pr> observed symmetrical relative to the horizontal boundary layer. By increasing h or decreasing the boundary layer _to a flow rate distorted - manifest effect on lift forces him.

The absence of influence on the combustion process when determining V _{ol of the} horizontal walls of the channel is indicated by the fact that the combustion limit by oxygen concentration (C _ol ), determined in a flat channel, practically coincides with the value of C _ol measured in open space.

This invention provides a definition of the main indicator of fire hazard of materials for orbital flight conditions - the burning limit of the flow rate in zero gravity (V _CR ) when exposed to acceleration of gravity.

 This development is of great practical importance, because it does not require the use of such material costs as are necessary for conducting experiments in true zero gravity (in freely falling containers, in laboratory aircraft, at space stations).

 The invention makes it possible to implement the latest developments in the field of fire safety of manned pressurized spaceships, in particular long-term orbital stations, interplanetary ships, etc., which are long in orbital flight.

Claims (1)

A device for determining the burning limit of materials by flow velocity for zero gravity conditions, comprising a combustion chamber with plane-parallel walls with a sample of the test material installed in it, characterized in that two opposite walls of the combustion chamber are installed perpendicular to the gravity vector, one of which is made with the possibility of plane-parallel movement while the height of the combustion chamber when determining the combustion limit of materials for zero gravity and depending on the parameters of the working gas atmosphere in which the material to be used is set based on the ratio

where P = P _o / P _cf - the term characterizing the pressure of the gaseous medium;
C _ok - volume fraction of oxygen in the gas medium;
g is the acceleration of gravity, cm / s ² ;
K _d is the empirical coefficient;
P ₀ - atmospheric pressure, MPa;
P _cf - gas pressure in the experiment, MPa.

Images