US3799812A - Transpiration radiometer - Google Patents

Abstract

An annular edged-cooled transpiration radiometer for measuring the total radiation incident on the walls of a circular tube containing a flowing plasma. The device is an annular, tube-like device and comprises a sensing means, a cooling means and a transpiration gas means. The device separates radiation from convection by blowing cold gas through a porous section of the circular tube wall. The alternative embodiment is a spot radiometer comprising a porous disk supported on the end of a cylindrical-shaped gas transpiration means which is also proved with a cooling means. The spot radiometer is suitable for installation into a duct or enclosure and is designed for "energy balance" operation.

Description

Mamh 1974 J. MOFFAT ETAL 3,79

TRANSPIRATION RADIOMETER 2 Sheets-Sheet 2,

Filed Aug. 22, 1972 United States Patent 3,799,812 TRANSPIRATION RADIOMETER Robert J. Molfat, Palo Alto, Calif., Bruce D. Hunn, Potsdam, N.Y., and Joseph Galate, Houston, Tex., assrgnors to the United States of America as represented by the Secretary of the Navy Filed Aug. 22, 1972, Ser. No. 282,749 Int. Cl. H01v 1/04 US. Cl. 136-213 9 Claims ABSTRACT OF THE DISCLOSURE An annular edge-cooled transpiration radiometer for measuring the total radiation incident on the walls of a circular tube containing a flowing plasma. The device is an annular, tube-like device and comprises a sensmg means, a cooling means and a transpiration gas means. The device separates radiation from convection by blowing cold gas through a porous section of the circular tube wall. The alternative embodiment is a spot radiometer comprising a porous disk supported on the end of a cylindrical-shaped gas transpiration means which is also provided with a cooling means. The spot radiometer is suitable for installation into a duct or enclosure and is designed for energy balance operation.

BACKGROUND OF THE INVENTION Field of the invention.-The subject matter of the present invention relates generally to an improved transpiration radiometer and more particularly to transplration radiometers that can detect radiant heat flux in a multitude of environments.

Description of the prior art-Considerable interest in heat transfer from internally flowing high temperature plasmas has been directed to aerospace applications, particularly in the study of the combustion process and high temperature transfer as it relates to rocket engine combustion chambers and flow passages or any other equivalent environment. At these plasma temperatures the radiative mode of heat transfer is usually significant requiring its inclusion in design calculations for heat loads on the containing walls of any of the above described environments. Due to coupled nonequilibrium effects, however, the generally available finite-difference type calculation systems available for plasma flow are unable to predict accurately the total radiative heat fiux to the contaimng walls. Since, in a plasma, this radiation reaches the wall together with mass-associated energy transfer by way of convection and ambipolar diffusion, it is necessary therefore to separate the mass-associated from the non-mass associated energy transfers to examine the radiant portion along the surface walls.

The surface walls exposed to high temperature environments frequently need thermal protection to ensure against failure. This type of protection is applied to situations such as cooling by transpiration, ablation, liquid cooling, slot cooling, etc. The optimum design of such protective systems is sensitive to the type heat load which is to be encountered' For example, relatively modest amounts of transpiration flow will suffice to substantially reduce the convective portion of the heat load to a surface, but will not at all affect the radiant portion. Similarly, it is desirable to know the distribution between radiation and convection heat load on the walls of a combustion chamber in order to predict the effects of changing fuel, for example, on the operating temperature of the walls. Products of combustion from some fuels radiate more than others. Classification of fuels, according to emissivities of their products, facilitates prediction of fuel interchangeability. Such classification requires measurement of the amount of radiation from the products.

3,799,812 Patented Mar. 26, 1974 The most commonly used class of radiation measuring sensors is that which absorbs the incident radiation into a solid and measures the resulting heat transfer or energy storage. Such devices can be subdivided into three groups: steady state, transient, and quasi-steady devices. This classification relates the maximum application time to the characteristic time of the sensor. Steady state devices are exemplified by the water-cooled thin disk Gardon gauges in which the temperature distribution of the thin watercooled disk is related to the total heat transfer to the disk. Such a device can operate indefinitely yet has a relatively short characteristic time. Transient devices are those re-' lying upon a change in energy storage to produce a signal, such as slug calorimeter. Quasi-steady describes the behavior of an uncooled Gardon gauge wherein the device will function for many times the duration of its characteristic time, but will usually fail by overheating when its heat-sink temperature exceeds some safe structural limit.

Devices of the above classification all respond to their total heat input, that is, radiation plus convection. Efforts to separate the radiation from the convection have previously concentrated on the use of low emissivity coatings to suppress the radiation or protective windows to suppress the convection. Neither approach is satisfactory in the presence of residual products of combustion since the characteristics of both the coatings and the windows may change with exposure of the environment.

SUMMARY OF THE INVENTION Briefly, the present invention is an annular edge-cooled transpiration radiometer for measuring the total radiation incident on the walls of a circular tube containing a flowing plasma. The device is an annular tube-like device and comprises a sensing means, a cooling means and a transpiration gas means. The device separates radiation from convection by blowing a cold gas through a porous section of the circular tube wall. The alternative embodiment is a spot radiometer comprising a porous disk sup ported on the end of a cylindrical-shaped gas transpiration means which is also provided with a cooling means. The spot radiometer is suitable for installation into a duct or enclosure and is designed for energy balance operation.

It has been found that convective heat transfer to a surface can be substantially eliminated by forcing gas through the surface in sufficient amounts to blow off the boundary layer. This eifect forms the basis for the unique present invention and its alternative embodiment.

The present invention and its alterative embodiment will overcome the aforementioned problems with regard to windows and coatings by separating the radiation and the convection through control of the surface convection by strong blowing (transpiration) through the boundary ayer.

When fluid is injected through a porous surface at a sutficient rate, the mainstream flow is essentially uncoupled from the surface, that is blown ofi. The surface heat transfer coefficient is reduced to zero and only radiant energy can reach the surface. The transpiration fluid thus forms an ideal window to protect the radiometer from convective inputs.

In high intensity plasma application, however, the incident radiative flux level is quite high and the plasma would be adversely effected by too high a blowing rate. This requires operation in the edge-cooled mode to protect the annular porous sensor without disrupting the mainstream fiow. Some plasma applications require radiation measurement at the wall of a circular tube of a small diameter. As a direct extension of the energy balance protective concept, an alternative embodiment with an annular edge-cooled design is provided to operate in the high intensity plasma environment.

Moreover, the unique invention and its alternative embodiment otters two principal advantages: first, the transpiration flow rate can be optimized for boundary-layer protection alone and, second, the main temperature gradients in the porous elements will be lateral rather than axial, simplifying the problem of measuring the temperature of the element.

The spot type radiometer described above is suitable for studies in air-breathing and rocket-engine-cornbustion chambers and flow passages. The annular design, operated in the edge-cooled mode may be used to measure radiation from a confined argon plasma. Both types of radiometers are high intensity devices, but the same sensor principle could be used to measure low radiant flux levels.

STATEMENTS OF THE OBJECTS OF INVENTION A primary object of the present invention is to provide a transpiration flow radiometer which will optimize the flow rate for boundary layer protection.

Another object of the present invention is to provide a transpiration radiometer with temperature gradients in the porous element in the lateral direction rather than axial to simplify the problem of measuring the temperature of the selected element.

Another object of the present invention is to provide a transpiration radiometer which will separate the radiation and the convective heat flow by controlling the surface convection by blowing or transpiration through the boundary layers.

Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross sectional view of the assembled annular edge-cooled radiometer parts being broken away for illustrative purposes;

FIG. 1A is a top section view of the assembled annular edge-cooled radiometer of FIG. 1;

FIG. 2 is an isometric view of the porous sensor for the annular edge-cooled radiometer, illustrated in FIGS. 1 and 2; and

FIG. 3 is a cross sectional view of the spot probe construction for an alternative embodiment of an edge-cooled radiometer.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Before describing the unique radiometers, a discusion of the theory of the transpiration radiometer is necessary. By forcing gas through surface area in suflicient amounts to blow-ofi, the boundary layer essentially eliminates convective heat transfer to the surface area. The effect of blowing through a porous surface is twofold: First, it causes changes in the boundary layer flow over the surface to reduce the gas transport coefficients; and second, it extracts energy from the porous surface, cooling it. Generally, the higher the mainstream mass velocity (pV) the higher will be the blowing required to uncouple the surface from the main flow, so that the strong blowing causes the surface to be entirely decoupled from the flow, to leave only the radiative input. A probe using this principle would permit direct measurement of the total hemispherical radiation incident upon a surface in the presence of strong convective flow. From an analysis of this situation there are two clearly defined modes of operation for a transpiration radiometer, edge-cooled and energy-balanced. There is a third or intermediate blowing mode. However, this blowing mode should be avoided since the centerline temperature becomes dependent on both the edge temperature and the transpiration gas inlet temperatures and interpretation of the signal becomes extremely complex for a workable device. The edge-cooled mode leads to radial distribution of temperature in the devices sensing means with the sensing signal derived between the centerline of the sensing means and edge of the sensing means. The blowing parameter is small compared to unity for this type of sensor device. Alternatively the energy balance mode (used in some prior radiometers) leads to an axial distribution of temperature in the sensing means with the signal derived between the temperature of the sensing means and the inlet temperature of the transpiration gas. The spot radiometer may be used for this configuration. The theory of operation will become apparent when the structural features of the spot and edge-cooled radiometers are hereinafter described.

Referring to FIGS. 1 and 1A, the annual edge-cooled radiometer 11 is made of two thin circular plates 13 and 15 which may be made of copper or any other metal with high thermal conductivity. Plates 13 and 15 are radially slotted on one side for form transpiration slots or gas flow channels 17 for the transpiration gas flow. Plates 13 and 15 are milled on the side opposite channels 17 to form cooling water channels 19 and 21. A /2-inch-diameter hole forms the plasma flow center tube 23 through which the plasma flow to be sensed is directed. The hole is tapped through each plate at its center and plates 13 and 15 are secured together around the porous sensor element or ring 25 in a standard manner. The radiometer 11 may be attached to an arc-jet generator nozzle, or the like, to measure the total thermal radiative flux incident on the walls of the tube of the nozzle through which the plasma is flowing.

Referring to FIGS. 1 and 1A, the assembled top plate 13 and bottom plate 15 include cooling water channels 19 and 21, respectively, on the outer sides of plates 13 and 15. The sealed water cooling chambers 19 and 21 are formed when covered plates 72 and 73 are positioned on the top plate 13 and bottom plate 15, respectively. Both plates 13 and 15 include a set of 18 radial slots 17 located on the inner sides of plates 13 and 15. When plates 13 and 15 are secured together, slots 17 form hollow channels to direct the transpiration gas flow uniformly to the porous surface of sensor 25 and subsequently to the center hole or tube 23. Slots 17 are each about 0.040 inch wide by 0.125 inch deep and are spaced about every 20 around the circumference of plates 13 and 15. Each slot is about 1 /2 inches in flow length providing effective heat exchange from the inlet plenum 27 to the end of the slot opening into the porous directional ring 29 which is spaced somewhat from sensor 25. It should be noted, however, that the porous directional ring 29 is neither necessary nor required for successful operation of radiometer 11 and is merely used for somewhat better flow distribution and control. When plates 13 and 15 are secured together, the recessed circular plenum 27 region is formed at the outer edge of secured plates 13 and 15. This serves to distribute the incoming transpiration gas circumferentially around the plenum 27. On the upper surfaces of plates 13 and 15 and opposite radial slots 17 are cooling water channels 19 and 21, respectively. Channels 19 and 21 are interrupted in the center by center tube 23. It should be noted that FIG. 1A illustrates a view taken from the top of plate 13. However, due to the similarity of plates 13 and 15 an illustration of plate 13 is deemed to suffice for a discussion of plate 15. The tube 23 is about /z-inch in diameter with a wall thickness of about 0.100 inch at the bottom of each water channel 19 and 21 around the center tube 23.

Referring to FIGS. 1 and 2, four holes about 0.030 inch in diameter are drilled at intervals; two of the holes are located in channel 19, apart; two of the holes are located in channel 21, 180 apart. The four holes are drilled to accept thermocouples 57, 59, 61, and 63. Thermocouples 57, 59, 61, and 63 are inserted into the four holes. Thermocouples 57, 59, 61, and 63 may be secured into holes by Thermon cement or any other potting material of similar qualities. The thermocouple leads can be extended out through the water inlet ports 65, 67 or they can be extended out through the water outlet ports 69, 71,

or any other acceptable means. To form Water inlet ports 65 and 67 and water outlet ports 69 and 71 a fit-inch diameter holes is drilled into the ends of channels 19 and 21 and a length of copper tubing is brazed into the recess of each hole. The cooling water inlet ports 65 and 67 are connected to any standard continuous water supply. The cooling water is exhausted through water outlet ports 69 and 71 and could be recycled if desired.

Water channels 19 and 21 are enclosed by thick cover plates 72 and 73, respectiv ly, Which fit into the recess around the channels 19 and 21. A thin layer of potting material may be used to form a gasket beneath coverplate 72 and 73 and secured in a standard manner.

Referring to FIGS. 1, 1A and 1B, the manifold ring 75 is a brass ring of a rectangular cross sectional area with an about 5-inch inside diameter and about a 6- inch outside diameter and fits into recess 77 of plate 13 and recess 79 of plate so that when plates 13 and 15 are secured together a plenum region 27 is formed just inside the ring 75 for circumferential distribution of the transpiration gas. Four gas inlet holes or ports 81, 83, 85, and 87 are drilled radially at 90 intervals through manifold ring 75 where the copper tubing is brazed into the recesses and around holes or ports 81, 83, 85, 87. The transpiration gas such as argon, or the like, enters the plenum 27 through ring 75 from a pressure regulated gas storage supply via polyfiow tubing attached to each of ports 81, 83, 85, and 87. O- ring seals 89 and 91 fit into grooves 93 and 95, which are located on both sides of manifold ring 75. O- ring seals 89 and 91 cause manifold 75 to seal itself between plate 13 and bottom plate 15 when radiometer 11 is assembled.

Referring to FIGS 1, 1A, 1B and FIG. 2, both the top plate 13 and bottom plate 15 are recessed around the center tube 23 on the transpiration gas flow sides to form plenum region 27 for the distribution of the transpirator gas to the sensor element 25. The porous sensor element has an inside diameter of about /2-inch and a wall thickness of about 0.10 inch and rests in 0.010 inch deep grooves 97 and 99 located in the top plate 13 and bottom plate 15, respectively, to assure a centric positioning. Thus, sensor element 25 becomes part of the wall of tube 23 in which the plasma flows. Four Chronel-Alumel, or the like, thermocouples 49, 51, 53, and 55 are mounted at the inside face in the holes. The holes are drilled every 90 through porous sensor 25 at the midspan of sensor 25. Thermocouples 49, 51, 53, and 55 measure the centerline temperature. In addition, at two of the midspan thermocouple positions opposite each other are edge thermocouples 57, 59, 61, and 63 mounted at the inside face clearance holes. Edge thermocouples 57 and 59 are at the downstream location and thermocouples 61 and 63 are at the upstream location. The holes are drilled about 0.033 inch from the sensor 25 edges. All eight thermocouples can be formed by running the wires through porous wall of sensor 25 so that the Wires are as near as possible to being flush with the inside surface of the porous wall of sensor element 25, or any other convenient manner.

The eight thermocouple leads are brought out through grooves in the bottom plate 15 and small holes in the manifold ring 75, or any other convenient manner selected. A sealant may be used where the leads pass through manifold ring 75.

When the radiometer 11 is fully assembled, the plates 72 and 73 are positioned over plates 13 and 15, respectively, to form the sealed water cooling chambers 17 and 19.

The annular edge-cooled radiometer 11 operates as follows: When a fluid is injected, fluid or transpiration gas is injected through the porous sensor element 25 at a sufiicient rate, the mainstream plasma fiow is uncoupled from the porous surface. This phenomenon also occurs in fully developed laminar fiows in porous tubes and rectangular channels. Based on these concepts, the annualar edge-cooled transpirator radiometer 11 incorporates porous sensor element 25 of small axial length as part of the tube wall. The porous sensor element 25 is sandwiched between and is in good thermal contact with the adjacent water-cooled segments 19 and 21 of the wall of the porous sensor element 25. Thus, the majority of the input radiative flux which leaves the sensor 25 and is directed by conduction from around porous directional ring 29 to the edges of sensor 25 adjacent to the cooling water flow. The edge-cooled mode leads to radial distribution of temperature in the sensor element 25.

The temperature signal is derived from the difference between the temperatures at the centerline and edges of sensor element 25. A standard calibration is required to relate this signal to the incident flux. The inside diameter surface of sensor element 25 may be blackened with black paint, or the like, to provide a high absorptivity surface for the radiation.

Referring to FIG. 3, the spot-type transpiration radiometer 111 has a porous sensing disk 113 which is made of porous bronze about 0.360 inch in diameter and about 0.100 inch thick. Porous sensing disk 113 is attached into a water-cooled internally copper-finned headpiece 115 which is cooled with a high absorptivity material. The temperature signal is derived from two small diameter thermocouples 117 and 119 which are differentially connected. The junction of thermocouple 117 is attached to the backface of disk 113. The junction of the other thermocouple 119 is mounted at the edge of the porous disk near the water-cooling headpiece 115. The headpiece 115 is internally finned to enhance its cooling and is brazed to inner gas delivery tube 121. Cooling-water is directed to the water inlet port 133 through coolant manifold 139 and the inner annular passage 125 to cool headpiece 115. The incoming water sweeps over the finned backface 115a of headpiece 115 and returns through outer annulus 127 to water outlet port 135. This configuration isolates the transpiration gas and the headpiece 115 from possible thermal effects from the sides of probe 112.

The transpiration gas is directed from gas inlet port 137 into the gas delivery tube 121 and is directed to thermocouples 117 and 119 and then to the surface of porous sensing disk 113. The temperature signal to be derived is the difference between the temperature of the center of the disk 113, which is measured by thermocouple 117 and the temperature of the edge of the disk, which 1s measured by thermocouple 119.

Gas delivery tube 121 is sealed off from the water delivery system 23 which is comprised of the inner annular passage 125 and outer annulus 127. The positive and negative thermoelements 129 and 131 are extended out to the electrical terminals and attached with low thermal solder, or the like. This minimizes the EMf generated by temperature gradients through the electrical connectors. The entry aperture 142 is sealed off from the water coolant manifold 139 and chamber 140 by an O-ring seal 141. Application of the probe 112 requires a metered supply of transpiration gas such as dry air or nitrogen or the like as well as a supply of cooling-water. The gas can be supplied by any regulated gas supply means which is well known in the art. The cooling water may be supplied by any common supply means. The water supply means can be a closed loop circulation system attached to water inlet port 133 and water outlet port 135. The spot type radiometer 111 described above is suitable for studies in air breathing and rocket engine combustion chambers and flow passages or in any combustion processes and high temperature heat transfers where radiation and convection are both significant.

What is claimed is: 1. A transpiration radiometer to sense radiation emitted from adjacent flowing gases comprising in combination: (a) a porous radiation sensing surface having at least one edge and a side opposite said sensing surface said sensing surface being in contact with said flowing gases;

(b) at least one means for sensing the incident radiation emitted from said flowing gases operatively attached to said sensing surface;

(c) a means for cooling said sensing surface said cooling means located to cool said at least one edge of said sensing surface; and

(d) a means for supplying a transpiration gas to said sensing surface.

2. The device recited in claim 1 wherein said radiometer is an annular-donut shaped radiometer and comprises a transpiration gas chamber and a hollow interior aperture located within said chamber to furnish said sensing surface with transpiration gas, said sensing surface including a thermo sensitive means located in said chamber.

3. The device recited in claim 2 wherein said at least one sensing means is located flush with said sensing surface.

4. The device recited in claim 3 wherein said thermo sensing means comprises a plurality of said sensing means located and equally spaced in the surface portion of said sensing surface.

5. The device recited in claim 4 wherein the sensing surface includes an inner surface and an outer surface said inner surface of said sensing surface is cylindrically shaped and in contact with the axial flow of gases.

6. The device recited in claim 5 wherein said sensing means comprises a first, a second, a third and a fourth equally spaced sensing means circumferentially located at the midline of said cylinder and a fifth and sixth sensing means being axially spaced and located above and below each of said first, second, third and fourth midline thermocouples.

7. The device recited in claim 1 wherein said radiometer is a cylindrical shaped tubular and comprises an internal transpiration chamber, a porous circular disk device, sensing surface mounted at one end of said radiometer having an outside edge and at least one leading edge and a cooling means coupled around the outside edge of said sensing surface.

8. The device recited in claim 6 wherein said cooling means is an annular donut shaped cooling ring located circumferentially around the outer edge of said sensing surface.

9. The device recited in claim 7 wherein said sensing surface includes a plurality of sensing means located along said at least one leading edge and the center of said sensing surface.

References Cited UNITED STATES PATENTS 3,596,514 8/1971 Meiferd et al. 136213 X 2,666,799 1/1954 Barsy 136232 X 2,911,456 11/1959 Volochine 136-213 3,053,922 9/1962 SChunke 136224 3,531,989 10/1970 Webb 136-213 UX OTHER REFERENCES Croft et al., Trans. AS. M.E., 58, pp. 117, 120, 121 (1936).

CARL D. QUARFORTH, Primary EXmainer E. A. MILLER, Assistant Examiner US. Cl. X.R.

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