Test rig design

To ensure that the air and simulated exhaust mass flows matched, the same stream of air was used for both. In other words, the flow path was much like that in a real recuperated gas turbine. The only differences were that the compressor was the shop air supply; the burner was an electric heater; and the turbine was replaced with an orifice to simulate the pressure drop it would create. (A turbine would also create an enthalpy loss and a temperature drop, but since this would occur outside the recuperator, the difference was not important to the test. It would only have required the heater to warm up the air more, if a turbine had been used to drop the pressure instead of an orifice.)

Also like the engine, the recuperator was housed in a pressure vessel. Compressed air flows through the outside of the recuperator and gets heated, while exhaust flows through the recuperator’s internal channels. This tends to load most of its walls in compression, and tends to squeeze any leaking cracks closed, rather than pushing them open and causing them to grow.

The path of the air flowing through the rig can be traced by referring to Figure 7.3. Shop air first entered at the left side of the Side View diagram at 60-90 psi (4-6 bar) and flowed through a variable valve, which dropped the pressure and thereby controlled the flow rate. Second, it flowed through the mass flow meter (Omega FMA-A2323, range 0-100 standard liters per minute). This mass flow meter worked by slightly heating the gas and measuring its temperature rise, so it truly measured the mass flow rate, not the volumetric flow rate. Third, the air supply tube entered the pressure vessel through a Swagelok fitting that was actually located in the lid (bottom of the picture) rather than in the sidewall as pictured. It was drawn this way to avoid clutter and make the diagram easier to follow.

Fourth, the flow passed a thermocouple and an absolute pressure probe. All t/c’s were 36AWG Type N in this test. The wires were protected by PFA insulation in the low temperature areas and 1.2mm diameter rigid ceramic insulators in the hot zones.

Fifth, the air left the supply tube and flowed into the recuperator via a plenum at the air channel entrance, shown in Figure 7.1 (b). The recuperator was bonded into a cavity in a piece of rigid ceramic insulation material (Zircar SALI) that had been sealed carefully, outside and inside, with a two layer coating of Zircar ZIRPORCOAT and AS-CEM alumina-silica cement. The tube was bonded into the plenum cavity in this block with epoxy. The assembly was inspected carefully before and after the test, and appeared to be leak-tight. There was no discoloration of the epoxy visible after the test due to heat, nor was this expected.

Sixth, the flow exited the recuperator via a collection enclosure made from the same ceramic insulation, flowing past three thermocouples – one at the right side of the air channel exit, one at the left side, and one centered in the collection tube. From there, it flowed into a large plenum where there was a probe that measured recuperator air outlet static pressure.

Seventh, the air flowed through holes in a ~10cm tall stack of insulation. The holes held Super Kanthal A-1 electric heater wire coils from Duralite in place and ducted the airflow through them. After flowing through the heater, hot air was collected in a semicircular indentation in the insulation layer beneath it (visible in Figure 7.4 (d) and (e)), and entered a well-sealed tunnel in the insulation that was covered by a hard mullite plate.

Eighth, the air, which was at elevated pressure up to this point, went through the orifice plate, which caused a pressure drop, just as the turbine would create a pressure drop in the engine. From there, it flowed past the two exhaust inlet temperature thermocouples and the static pressure probe, though the recuperator exhaust channels, and out of the rig via the gasket plate and thick-walled tube pictured in Figure 7.4 (a) and (b) respectively. This small gasket was made from laser-cut Viton sheet, sized to match a ridged flange on the exhaust tube. The recuperator, gasket plate, gasket, and exhaust tube were all bonded together with silicone sealant. The integrity of the seal was checked at the beginning of the test. The author held his hand over the exhaust tube outlet, preventing the air from leaving the pressure vessel and causing pressure to build up inside it until he could no longer keep it from leaking out by hand. This was done twice, as can be seen in Figure 7.8 at around time=300 seconds.

The basic method of controlling the rig was to adjust the upstream air regulator valve until the desired mass flow rate was reached on the flow meter. This would create an elevated pressure in the vessel. As the rig was heating up, the pressure drop across the orifice would slowly rise. Since the exhaust outlet pressure was fixed (ambient), this caused an increase in back pressure, and thus a higher pressure inside the pressure vessel (upstream from the orifice plate).

The thermocouples, pressure transducers, and mass flow meter were connected to signal conditioning modules in a National Instruments CompactDAQ chassis, which in turn was connected to the data acquisition computer via USB. A LabView program developed by NRL Code 5712 engineer J. Smith and summer student G. Rancourt monitored and recorded all signals. Wiring was mostly done by physicist K. Goins. All welding was done by engineer M. Schuette, who also suggested the cooling system and supplied some of the parts. The author took the lead in designing and building the test rig, with assistance and significant contributions from the entire team, particularly J. Smith.

Figure 7.4. Test rig photos showing assembly sequence.

(a) Recuperator mounted in insulation block, with outlet plate and gasket bonded to the recuperator with

silicone sealant. These faced downward when installed. Exhaust inlet port at right was bonded to the

ceramic orifice plate with AS-CEM cement.

(b) Upside-down lid of pressure vessel that served as base of rig. Copper tube was for cooling water. Large pipe at the back is the recuperator exhaust outlet; to its

left is recuperator air inlet. Small tubes are pressure probes. Large flat circle at front is a wire pass-through.

(c) Rig base with castable ceramic material that covered the water cooling tubes and filled in the concave lid to create a flat surface. S-shaped tube is exhaust inlet pressure probe that protrudes through

orifice plate in (e).

(d) First insulation layer added. Exhaust outlet and orifice plate are at left. Air inlet is just below and to the

right. A copper tube ported air from there to the recuperator.

(e) Closeup of orifice plate and recuperator block, showing exhaust inlet thermocouples and pressure

probe bonded into the orifice plate, and air out thermocouples to the left of this.

Figure 7.5. Test rig photos before and after running the experiment.

(a) Recuperator block bonded onto the orifice plate; t/c’s and air outlet pressure at right. AS-CEM cement was being used to bond air outlet thermocouples and upper block. Shiny tube at rear is air inlet pressure.

(b) Main insulation block stack that holds the heater coils and creates the flowpath through it. Heater coils are energized. Blue RTV and AS-CEM cement were used to bond and seal this to the rest of the rig.

(c) Side view of rig stack showing inverted U-shaped copper air supply tube and smaller-diameter copper tubing

of the cooling water loop. Thermocouple wires are in front center. Pressure vessel and clamps showing at left.

NI CompactDAQ signal conditioning chassis at right.

(d) Complete stack with top insulation layer ready to be bonded in place with RTV. Next steps were to wrap stack

with copper wool to distribute cooling evenly; vacuum and clean dust out of gasket groove at edge of base; place

gasket in position; slide the pressure vessel can onto the assembly from above; tighten the clamps.

(e) Ready to begin test. Data acquisition system is at right; mass flow meter and air pressure regulator front/center; test rig at left; 1500W DC power supply (red

and black unit) at far left.

(f) Top view of rig after test. Top insulation layer has just been removed by cutting through blue RTV. No cracks or air leaks were found in any critical areas. Test was cut short by a cooling water leak and an intermittent ground