Last Updated: 9/19//2013
The contour plots of temperature computed by the cross-flow option in the HTT_Hx program correspond to looking at the radiator from the side. Radiators of this type were used in early automobiles and aircraft before engineers realized that there should be far more area on the air side to compensate for the lower heat transfer convection coefficients in air vs. that in liquids.
Detail of a Honeycomb Radiator
In manufacturing a honeycomb radiator like the one in this photo, both ends of the tubes (which in this case are about 3” long) were enlarged into a hexagon shape as seen here. A bundle of tubes (in this case about 150, but in some cases thousands) was bound together and soft brazed by dunking each end of the assembly in turn into a solder bath. The solder would rise about 3/8” into the gap between tubes by capillary action, but would not rise into the tubes themselves. Air flows in the inside of the tubes, while the liquid to be cooled (probably oil) flows in the gaps between them. A plugged tube and the tube failure that made it necessary may be seen at the bottom left. Radiator courtesy of B. Hosticka
installed on a Spad XVI
(displayed at the National Air & Space Museum Udvar-Hazy Center)
Honeycomb Radiator on President Woodrow Wilson’s 1919 Pierce Arrow.
vehicle at President Wilson’s birthplace
Cross-flow Heat Exchanger Mockup
This homemade mockup shows the flow patterns in a cross-flow heat exchanger with both fluids mixed. The hot (red) fluid and cold (blue) fluids flow between alternate sheets. The dimples keep the aluminum sheets separated and promote mixing. Since there are no barriers to lateral flow for either fluid, this geometry would be considered “both fluids mixed.” Note that the surface areas exposed to each of the two fluids is the same. Thus you would use this configuration when the two fluids are similar as in an air-air heat exchanger for waste heat recovery.
In this compact automobile heater core, the liquid (engine coolant) makes two passes. In the first pass it enters the hole at the top left and flows downward through the 12 flattened tubes. When the liquid reaches the bottom plenum, it is mixed before making the second, upward pass and coming out the hole at the top right. The liquid within the core itself is “unmixed.” You can look into either hole and see that within each of the 24 flat tubes (which you are seeing in edge view in this photo), the liquid is confined to discrete channels. Similarly the air (flowing normal to this photo) is unmixed because of the cooling fins. (And note that unlike in the “honeycomb” radiator above, the surface area exposed to air is far greater than that exposed to liquid.) The coolant sees fresh air entering on both passes. This exchanger thus corresponds to the fifth geometric configuration available with the HTT_hx program. The temperature contour plots for this geometry correspond to side views, and the two passes of the hot fluid are plotted with the first pass flipped and upstream of the second. The color plot below shows the orientation of the contour plots with respect to this exchanger.
In this partly “dissected” heater core plastic twisted-tape inserts are seen being used to increase mixing of the liquid on the tube side and thus reducing that contribution to the overall thermal resistance between the liquid and air. (Photo courtesy of Bo Hosticka)
The heat exchangers used with the Lake Source Cooling Project at Cornell are gasketed plate heat exchangers. Another type of plate heat exchanger is the welded plate heat exchanger. This type is not designed to be disassembled easily.
The Clinch River Breeder Reactor Plant was to have
been on the Clinch River some 12 miles southwest of
Schematic of CRBR Intermediate Heat Exchanger (IHX) The tubes in this single-pass, counterflow shell-and-tube heat exchanger were over 25’ (7.5m) long. The hot sodium coming from the reactor entered the IHX at about its mid-height, traveled upward through an outer annulus and then downward through the shell and out the bottom back to the reactor core. The intermediate sodium returning from the steam generator system entered at the top, flowed downward through the center core to the bottom and back up through the 2850 tubes. The three identical IHX’s were designed and fabricated by Foster-Wheeler.
Photo of CRBR IHX tube bundle under construction
In the photo of a small shell-and-tube heat exchanger seen below, only six of the 37 tubes are in position. When the other 31 are in place, then the single-segmental baffles act as barriers, thus forcing the shell flow to take a serpentine path through the shell. In this way that fluid (in this case a condensing refrigerant) flows at nearly right angles to the tubes and the resulting turbulence increases the shell-side heat transfer coefficient.
Heat pipe heat exchangers are sometimes used for air-to-air energy recovery systems. These devices involve three fluids: the two air streams between which heat is being transferred and a third fluid sealed within the multitude of heat pipes making up the unit.
In a typical application exhaust air and fresh air are flowing in opposite directions, i.e., in a counterflow arrangement, in adjacent ducts with the unit spanning the cross-section of both ducts. In the winter (as seen above in the schematic) heat transferred from the warm air being exhausted provides the energy to evaporate the working fluid in the sealed heat pipe. That vapor flows to the other end, where it condenses, giving up the heat to the incoming fresh air. The condensed liquid flows back to the warm end to complete the cycle. In the summer the operation is reversed. The warm, but fresh, air entering the building is pre-cooled by transferring heat through the heat pipes to the cool, but stale, exhaust air leaving. To compensate for the low heat transfer coefficients with gases, the outside surfaces of the heat pipes are aggressively finned.
Plate Fin Heat Exchangers
A plate-fin heat exchanger has corrugated spacers between the parting plates separating the two fluids. In the mockup of a cross-flow exchanger in this photo, offset strip fins are seen guiding the fluids in adjacent layers in perpendicular directions.
Rotary regenerators may be used to reclaim waste heat as in a fossil-fueled powerplant or in conjunction with the Brayton cycle for gas turbines. In the very large regenerator seen in the schematic, the hot waste stream is flowing downward to the left of the circumferential seal. The fresh air is flowing upward (countercurrent) to the right of the seal. On the exhaust side heat is deposited in the matrix forming the rotating element. When that section of the wheel gets to the fresh air side, the stored heat is released to the fresh incoming combustion air. “Each element weighs between 2 and 4 tons, and with the rotor and seals, it's a 200 ton, 20 ft diameter rotating assembly turning at about 1.5 rpm. Incoming air is heated from 100 degrees F to 500 degrees F, and the exiting flue gas drops from 600 degrees F to about 250 degrees F. We do periodic inspections on the elements to ensure that the corrugated steel heat transfer surface is in good shape...” (Dan Moses, Potomac Electric Power Company).
In the Brayton cycle with regeneration, waste heat that otherwise would be discharged to the atmosphere, is used to preheat intake air which has been compressed before it gets to the combustor. The experimental Chrysler turbo car of the 1960’s and 70’s incorporated such a system:
Gases leaving the power turbine (which rotated at speeds as high as 70,000 RPM) passed through either of the two slowing rotating regenerators (which rotated at much more leisurely speeds up to a maximum of 31 RPM). One-hundred eighty degrees away, the ceramic material making up the rotor gives up the heat just stored to the relatively cool air coming from the compressor. In this way the heat needed to raise the charge temperature from 430oF to 1200oF (see schematic above) was saved. The hardware including the regenerators may be seen in this exploded view and in the front view of a mockup. (Photos courtesy of T.C. Scott.)
Incropera, F.P., and DeWitt, D.P., Fundamentals of Heat
and Mass Transfer, 5th Ed., Wiley,
Shah, R.K. and Sekulic, D.P., Fundamentals of Heat Exchanger Design, Wiley,
Des Champs Laboratories Incorporated, Heat Pipe Heat Exchanger, Bulletin HPHX1193/1.0, 1993.
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