1. Technical Field
The present invention relates, in general, to packed bed heat exchangers and, in particular to a system and method for their use as a compact heat source for short duration hot gas flow applications.
2. Background Art
The packed bed, regenerative heat exchanger has been in use for over a century and its heat transfer and fluid flow characteristics are well defined, For a bed with spherical pebbles, the pressure drop is given by EQU Delta P=f(L*G.sup.2 /D.sub.p *Rho)
where L is the length of the bed, G is the mass flow rate per unit cross-sectional area of the bed without the pebbles, D.sub.p is the diameter of the pebbles, and Rho is the average gas density in the bed.
These heater have been used in commercial applications in the metals industry for heating air for iron smelting. These heaters consist of cylindrical steel vessels, about 50 to 100 feet high, which contain a cylindrical stack of cored ceramic bricks, (instead of ceramic spheres), surrounded by a low thermal conductivity cylindrical insulating layer. Gas combustion products at temperatures of up to 1650 Celsius pass through the nominal 1/2 inch diameter cored passages from the top to the bottom of the heater. After a period of several tens of minutes, the combustion gas flow is shut off by closing a valve, and cold air is forced upward through the passages in order to heat the air to near 1650 Celsius. Three to four of these heaters are cycled sequentially in order to yield a nearly constant flow of hot air from these heaters.
In the 1960's extensive R&D began on the development of high temperature magnetohydrodynamic (MHD) power generation systems. Thermodynamically MHD power systems are identical to gas turbine Brayton power cycles in which the MHD generator replaces the gas turbine-generator part of the cycle. Power is produced when a high temperature electrically conductive gas passes through a transverse magnetic field. By Faraday's law of electromagnetic induction, an induced electric field is produced in the gas and power can be drawn by means of electrodes inserted orthogonally to the magnetic field and gas velocity vectors on opposing walls of the MHD generator channel. To render the gas electrically conduction, it is necessary to heat it to the range of 2000K to 3000K. At the lower temperatures, the induced magnetic field can enhance the gas electrical conductivity to above its equilibrium value based on the gas temperature. This process is practical only in the noble gases, such as helium or argon, and in diatomic gases with homopolar molecules, such as hydrogen and nitrogen. Concentrations of other molecular gases above the range of several tenth's of one percent will quench this non-equilibrium conductivity effect. Therefore, a means of indirectly heating these gases to the 2000K range must be used, and ceramic cored brick or ceramic spherical pebble bed heaters have been used since the early 1960's. For research purposes, these heaters were sometimes heated with electrical elements placed in the bed matrix. In addition, some researchers used graphite as the packed bed material due to its higher thermal storage capacity. (. R. Decker, M. A. Hoffman, & J. L. Kerrebrock, "Behavior of a Large Non-Equilibrium MHD Generator", AIAA J., Vol. 9. No. 3, March 1971, pp. 357-264) ("Ref. 1"). Others used ceramic pebbles or ceramic cored bricks as the heat storage elements. In addition, combustion gases were used to heat the bed to operating temperature, at which time combustion ceased, the bed was evacuated and the MHD test gas, argon or helium, was blown upward through the bed for periods of about 1 minute. (C. S. Cook,, "Current Experimental Results from Operation of the GE Closed Cycle Ceramic Regenerative Heat Exchanger", Proceedings 15th Symposium on Engineering Aspects of MHD, U. of Pennsylvania, Philadelphia, Pa., May 24-26, 1976, p. VII.4) ("Ref. 2"). In the latter case, the objective of the test was to evaluate the operation of the heat exchanger for future design in a commercial MHD system. For this purpose, a temperature profile increasing from several 100 C. at the bottom of the bed to 1725 C. at the top of the bed was produced by the combustion gas. This temperature profile assures most efficient heat transfer from the combustion gas to the bed and then to the argon gas. The combustion gas entered the bed on top and exited at the bottom. The MHD test gas, argon, entered the bed at the bottom and exited at the top toward a simulated MHD channel. The period of constant heated gas temperature output is determined by the region at the top of the bed that has been heated to the peak temperature. In other words, if, for example, the top one foot height of the bed was heated to 1725 C. and the remaining bed height has a gradually decreasing temperature profile, the argon gas outlet temperature would remain constant until the thermal cooling wave due to the argon has reached the bottom of the 1 foot section at the top of the bed. The maximum time of the constant temperature gas pulse is determined by the thermal energy stored at constant temperature, namely 1725 C., in the solid bed material, which in this example is 1 foot. A longer duration constant temperature gas pulse from a heater having the same diameter bed requires a longer section of solid bed material at constant temperature. Conversely, a shorter constant temperature gas pulse requires less bed material. In other words, the maximum time of the constant temperature gas pulse is determined by the mass of solid material at the peak temperature in the bed. This relationship of maximum constant temperature pulse length to quantity of bed material is well known to designers of storage heat exchangers. It is thus obvious that to achieve a constant temperature for a long time period in this mode of operation, an extremely high bed height is required. This is especially the case when the added restriction of a low gas pressure drop is added to the bed design criteria, as discussed below.
The plasma physics and fluid mechanics of MHD generators are described in a numerous books. The ceramic heaters are also described in a number of texts.
It is well known that for high thermodynamic efficiency in a gas Brayton cycle, the gas pressure losses must be minimized. This dictates the use of very large packed bed heater exchangers, irrespective of the use of ceramic cored bricks or spherical pebbles. In Cook's packed bed, cored brick ceramic heat exchanger, a thermal output of only 3000 kW (thermal) for a 1 minute blowdown in argon at 10 atmospheres pressure, required a central core of about 16 inches in diameter and 10 feet high. This core, weighing over 1 ton, was surrounded by a thick ceramic insulating layer sufficient to maintain the steel shell as a safe temperature. The height of the bed and its gas passages cross-section are also dictated by the need to prevent the high pressure argon flowing in the upward direction from fluidizing the upper ceramic cricks or pebbles.
Due to the massive size of these beds, whether of graphite or ceramic, (e.g. alumina), they were used primarily for tens of a second to one minute duration high pressure blowdown purposes, and with the objective of simulating as closely as possible, an actual commercial MHD power generation cycle. As noted, all researchers forced the test gas to flow in the upward direction, which meant that a relatively low, (less than 10%) pressure drop was required to prevent fluidization of the bed. This also dictated a large gas passage cross-section in the packed bed.
Additional background art relating to the present invention is the use of shock tubes and shock tunnels to provide the high gas temperature flowing gas for MHD channel testing and wind tunnel testing. Due to the relatively high cost of conducting experiments that required very high temperature flowing gas sources, such as in non-equilibrium MHD generator research, shock tubes and shock tunnels were used. The shock tube provides a hot (2000 to 10,000 C.) gas source for periods of several 100 microseconds in the noble gases of interest in MHD power, (B. Zauderer, "Shock Tube Studies of Magnetically Induced Ionization", Phys. Fluids, Vol. 7, pp 147-9,{1964}) ("Ref. 3"). To increase the hot gas flow time to the range of 10 milliseconds, a shock tunnel was used in which the shock heated gas, after traveling down a long pipe was reflected from the downstream end of this pipe, bringing the high velocity gas to essentially stagnation gas conditions. A small nozzle placed in the end wall of the shock tube allowed outflow of the gas into the test channel, such as the MHD generator, for periods of up to 10 milliseconds. (B. Zauderer & E. Tate, "Performance of a Large Scale Non-Equilibrium MHD Generator", AIAA J., Vol. 9, No. 6, June 1971, pp. 1136-1143) ("Ref. 4"). These devices allow the performance of multiple tests per day at relatively very low operating costs. However, they have a relatively high first cost, and they occupy considerable space. For examples, in Ref. 4, a 10,000 kw thermal power, 10 millisecond duration, 1700 C. to 4000 C. argon and/or neon gas shock tunnel that was used for non-equilibrium MHD generator research, had an overall length of nearly 80 feet. The shock tube section was 1 foot in diameter and 50 feet long. Clearly these devices are very costly research tools and they have no potential for commercial applications as portable pulsed power sources. In addition, their limited test time eliminates them as a suitable source for study of hot gas-MHD generator wall thermal phenomena. Also, these devices are not suitable for testing low molecular weight gases such as helium or hydrogen, which are of major interest for non-equilibrium MHD application because their low molecular weight assures a very high power density in the MHD generator.
Additional background art relating to the present invention is the use of a metal fuel-oxidizer combustion heat source for MHD and wind tunnel applications. Zauderer has patented this system (B. Zauderer, Magnetohydrodynamic System and Method, U.S. Pat. No. 4,851,722, Issued Jul. 25, 1989) (Ref. 5). By using the liquid and solid products of metal-oxygen reaction to directly heat the MHD gas, such as helium or hydrogen, in the metal fuel combustion chamber, a very compact high energy density heat source is obtained for use in MHD generator and wind tunnel applications. Operation continues as long as the fuel feed is maintained. Due to ignition startup considerations, a practical lower limit for this device is a little under 1 second. Operation for periods in excess of a few seconds requires continuous removal of the metal oxide products of combustion in liquid form, as disclosed in Ref. 5. To provide a basis for comparison with the above packed bed heater size, a 10,000 kw thermal combustion chamber is about 6 inches in diameter and about 1 foot long. This compares with the 50 foot long shock tube, and the I ton, 10 foot high packed bed heater. Also, the oxygen and MHD test gas can be stored at high pressure (2,500 to 10,000 psi) to minimize storage volume.
A variation of this approach suitable for periods of 0.1 second duration is to use solid fuel-oxidizer pellets which can be chemically bound with hydrogen and the alkali metal seed, as the hot MHD gas heat source. Examples of such fuels are metal hydrides, such as zirconium hydride, and oxidizers such as ammonium nitrate. This heat source was tested for non-equilibrium MHD applications and is described in B. Zauderer, F. Rodgers, B. Borck, "Initial Experiments on a Chemically Heated Non Equilibrium MHD Generator", Proceedings 28th SEAM MHD Symposium, Chicago, Ill., Jun. 28, 1990, Chicago, Ill., Jun. 28, 1990). ("Ref. 6") A major disadvantage of these metal fuel heat source systems is safety considerations. Due to the danger of accidental explosions of the fuel oxidizer, the operation must be conducted under safety conditions suitable for explosion hazards, which substantially adds to the operating costs for test and commercial applications.
Additional background art relating to the present invention is the injection of the alkali metal seed into the hot MHD test gas stream. For multi-second and longer test periods, the alkali metal is injected through a positive displacement syringe either directly into the hot gas stream through various types of atomizers (W. J. M. Balements & R. H. Th. Rietjens "High Enthalpy Extraction Experiments with the Eindhoven Blowdown Facility" Proc. 9th Int.MHD Symposium, Tsukuba. Ibaraki, Japan, Nov. 17-21, 1986, Vol. II, pp. 330-340), ("Ref. 7"), or the liquid stream is injected into a chamber in which the alkali liquid is vaporized in a chamber and mixed with a small quantity of MHD test gas, (e.g. helium) and expanded into the main hot gas stream (M. Ishimura, et. al., "Engineering for Experiments of the Fuji-1 Facility", Proc. 9th Int.MHD Symposium, Tsukuba. Ibaraki, Japan, Nov. 17-21, 1986, Vol. H, pp. 351-358) ("Ref. 8"). The critical deficiency with these injection schemes is that in their reported form they are unsuitable for short duration tests of one second or less duration.