SE527154C2 - Internal gas heating device for e.g. space rocket micropropulsion systems, comprises heating coils located in central gas flow region - Google Patents

Abstract

The device comprises heating coils located in the central gas flow region, enabling the cold gas to be heated up without significant heat loss via the nozzle chamber walls.

Description

20 25 30 35 527 154 Z micro fi propulsion systems exhibit superior performance, especially if proportional control is used.

The way to increase the specific pulse is to raise the gas temperature. It is a simple square relationship between gas temperature and specific impulse. If the temperature is increased fi than 300 ° K to l200 ° K, Isp doubles and at 2700 ° K Isp is three times higher, nmt 200 sec. for nitrogen gas.

There are two difficulties in implementing such a temperature increase in a micro drive system. The first problem is that silicon, which is used to micromechanically produce the small structures required, conducts heat very well, which leads to significant heat losses.

The second problem is the small dimensions involved, under certain dimensions laminar fate dominates, this causes problems with gas mixing.

In the current warming, the disadvantage of fate has turned into a significant part. If an electric heater is placed inside a gastronomic port area at a given distance from the surrounding walls, the near-fate of the layers closest to the wall will act as an insulation for the hot gas in the center.

The flow rate has a parabolic distribution (1 1) in a tube (10) with zero velocity at the wall, see Figure 1. This means that the amount of “cold” gas expelled through the nozzle is small compared to the hot main stream.

Since the wall temperature in the pipe (10) is significantly reduced, the heat loss due to thermal conduction will be reduced accordingly. 4 BRIEF DESCRIPTION OF FIGURES Figure I ger gives the rate of fate of a gas in a pipe.

Figure 2 is a section through a double membrane heater.

Figure 3 is a top view of a membrane with a typical heating element pattern.

Figure 4A is a view of a cylindrical filament heater.

Figure 4B is a view of a conical filament protector.

Figure 5 shows a double filament arrangement.

Figure 6 illustrates manufacturing methods for a filament.

Figure 7 shows the result of a temperature distribution simulation. 10 15 20 25 30 35 40 527 154 - Figure 8 is a diagram showing the radial temperature distribution. 5 DETAILED DESCRIPTION OF THE INVENTION In the prior art, there are two direct difficulties in introducing a sharp temperature rise of the gas temperature into a micro-propulsion system. The first is that silicon, which is used to micromechanically produce the small structures required, conducts heat very well, leading to significant heat losses. Silicon also loses its good mechanical properties at elevated temperatures. Already at 1000 ° K the mechanical strength decreases and at 160 ° K the material melts. The second problem is the small dimensions involved. Under a certain size, laminar fl fate dominates, which means that gas mixing and efficient heat transfer fi from an external heater is made more difficult.

In the present invention, the disadvantage of laminar fate has been turned into a significant advantage. If an electric heating element is placed inside the gas transport duct at a certain distance from the walls, the laminar fl in the layer near the wall will serve as a thermal insulation for the hot gas in the center of the duct. zero speed at the wall, see Figure 2. This means that the amount of cold gas, which is forced through the nozzle, is small compared to the hot main stream.

Two different types of heating elements are conceivable, membrane heaters and filament heaters.

Both types are briefly described in the following.

In an membrane membrane heater, the heat transfer channel consists of two bonded silicon wafers, each with an anisotropically etched channel (30), one or more non-thermally conductive membranes (22) which mechanically support the electrical Figure 3 is a top view of a membrane membrane. The membrane (31) consists of a thick oxide layer with a thermal conductor pattern deposited on the oxide. After this, the channel is created in a second etching step. The heat conductor pattern is connected to a terminal on the silicon body.

One method of manufacturing the heat conductor pattern may be laser assisted deposition of tungsten through the process WóF + 3H2 4- -> Wß) + 6HF (,) _ Another design concept for the heater may be internal filament heaters, which are suspended centrally in the heat transfer chamber. The heating coils can be cylindrical or comic, figure 4.

The conical design is probably more efficient because each relief lap is directly exposed to the gas flow. The players are suspended centrally in the gas duct and e.g. welded to the electrical terminals using FIB wol fi arn deposition.

The efficiency and system reliability can be increased by mounting more than one heater in the heat transfer chamber and connecting them in parallel to the power terminals. An image of a two-coil arrangement is shown in Figure 5.

The most promising filament material appears to be diamond-like carbon with a thin tungsten coating of tungsten applied by means of a CVD process.

The manufacturing process for producing the diamond-like carbon is laser assisted deposition of carbon from ethane through the process C2H4 -> 2C (,) + 2H2 - The deposition process is illustrated in Figure 6. 10 20 25 30 35 527 154 ff Required heat output and heat losses through the walls are studied using the rivet element method. The result of the simulation shows how much power is required and how large the protective losses are through the walls. Temperature and velocity distributions are also partial results.

Numerically, an axial symmetrical system has been studied, the channel diameter is 220 μm and the glow diaphragm 120 μm. The length of the filament deflector is 1 mm. The purpose of the simulation is to mmm what effect is required to double the specific pulse. This requires a fourfold increase in the gas temperature from 300 ° K to 1200 ° K. 1 mN. In the synchronization, the input power is set to 1.24 W.

The electric heaters are calculated to be 0.43 W and the average output temperature is 139l ° K, see Figure 7. The limited factor in this optimization is that the peak temperature in the heating coil increases with the minimum coil diameter, in this example maximum temperatures are 2680 ° K.

As shown in Figure 7, temperature increases are greatest near the central axis. This is illustrated in Figure 8 where the output temperature is plotted as a function of the radial distance from the central axis.

Claims (1)

1. 0 20 25 30 35 527 154 S 6 PAn-: mmznv The following patent claims characterize the invention: A method of heating the gas in a desert (II) through a tube (10) of microscopic dimensions to a higher part temperature than the maximum material wall of the tube. The method centrally in the pipe through a heating element heated gas and the use of two phenomena. First of all, the pipe wall is insulated by the lamina gas forced through the small dimensions, which prevents the hot gas from coming into contact with the pipe wall. The tooth decay differs from the niluoscale relative to the thick gray grain of stagnant gas even to the insulation. A system for heating a gas-desolate genomic tube on a microscale by means of electrically energized systems of electromechanical heating elements (33, 53) which are resistively heated. A system according to sea 2, characterized in that said heating element is suspended by non-heat-conducting membranes or by poorly heat-conducting lcra lum lumens, in order to reduce thermal conductivity. A system similar to a crawler is characterized by a water transfer chamber connected to a nozzle (54). A system according to claim 4, characterized in that said heat transfer core has a heat-exerting coating on the surface and reduces the loss of heat. A system according to claim 4, characterized in that said heating element consists of a number of heating elements connected in parallel. A system according to claim 6, characterized in that said heat exchanger may have a dense density or heat transfer capacity depending on its physical location in the heat transfer chamber.