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RESEARCH PAPERS: Melting and Solidification

Frost Growth in Regenerative Wheels

[+] Author and Article Information
Wei Shang

Hong Chen

Department of Mechanical Engineering, University of Saskatchewan, 57 Campus Drive, Saskatoon, S7N 5A9 Canadahong-chen@utulsa.eduPetroleum Engineering Department, University of Tulsa, 2950 E Marshall Street, Tulsa OK 74110 USAhong-chen@utulsa.eduDepartment of Mechanical Engineering, University of Saskatchewan, 57 Campus Drive, Saskatoon, S7N 5A9 Canadahong-chen@utulsa.edu

Robert W. Besant

Department of Mechanical Engineering, University of Saskatchewan, 57 Campus Drive, Saskatoon, S7N 5A9 CanadaPetroleum Engineering Department, University of Tulsa, 2950 E Marshall Street, Tulsa OK 74110 USADepartment of Mechanical Engineering, University of Saskatchewan, 57 Campus Drive, Saskatoon, S7N 5A9 Canada

J. Heat Transfer 127(9), 1015-1026 (Apr 20, 2005) (12 pages) doi:10.1115/1.2005274 History: Received July 19, 2004; Revised April 20, 2005

Abstract

An experimental investigation was carried out for frost growth in a desiccant-coated regenerative wheel. The test facility was set up following ASHRAE Standard 84-1991R. Temperature, relative humidity, mass flow rate, and pressure drops were measured at each measuring station. Photos of frost within energy wheel flow channels show frost accumulation. The problem of frost growth within the narrow parallel flow passages of a regenerative heat or energy rotary wheel is formulated for a very cold-temperature ventilation application. Frost growth is assumed to grow as a porous media while the wheel is exposed to warm humid airflow on the exhaust side. While the wheel is exposed to cold dry airflow on the supply side, the frost is cooled but no frost grows. This cyclic frost growth and cooling process is continued with each wheel rotation. An analytical/numerical model is developed to simulate these frost properties over the depth of the wheel and as a function of time. Simulation results are used to interpret experimental data for the early stage of frost growth on a typical energy wheel with a cold supply air temperature of $−40°C$, a warm exhaust temperature of 20 °C and 40% relative humidity. Pressure drop measurements across a wheel taken for constant mass flow conditions revealed some very significant fluctuations of up to 100% of original pressure drop with a period ranging from 2 to 4 min for a wheel speed of 20 rpm. Each fluctuation in pressure drop is interpreted to imply a catastrophic failure of the outer frost layer sequenced over 1–2 min throughout the wheel followed by another frost growth period on top of a slightly thicker frost base.

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Figures

Figure 1

(a) Rotor of a typical regenerative heat exchanger and three different geometry matrices: (b) parallel surface; (c) hexagonal honeycomb; and (d) corrugated geometry

Figure 2

(a) Schematic diagram of the laboratory experimental test facility for frost growth within a regenerative wheel and instrumentation and (b) a schematic diagram showing the air flow through a regenerative wheel

Figure 3

Frost growth measurement system: (a) frost photosystem with a boroscope and (b) static pressure drop measurement and laser detector system

Figure 4

Photos of some energy wheel flow channels: (a) before frost growth (amp.factor=10); (b), (c), and (d) after frost growth (amp.factor=12),T1=−40°C,ϕ1=50%,g1=2.41kg∕(sm2),Δp1−2(t=0)=35Pa,T3=20°C,ϕ3=40%,g3=2.41kg∕(sm2),Δp3−4(t=0)=38Pa, and wheel speed is ω=20rpm

Figure 5

Temperature data at stations 1–4, g¯a=2.41kg∕(sm2)

Figure 6

Sensible effectiveness of the regenerative wheel vs time during frost growing

Figure 7

Humidity ratio data at stations 1–4 with temperatures and flow rate as in Fig. 5

Figure 8

Dry air mass flux vs time with air flow properties as in Figs.  57

Figure 9

Pressure drop across the wheel vs time for (a) the supply air side; (b) the exhaust air side; and (c) the laser beam sensor: T1=−40°C,ϕ1=50%,g1=2.41kg∕(sm2),Δp1−2(t=0)=35Pa,T3=20°C,ϕ3=40%,g3=2.41kg∕(sm2),Δp3−4(t=0)=38Pa, and wheel speed is ω=20rpm

Figure 10

Two typical cycles of pressure drop across the wheel vs time for (a) the supply air side, (b) the exhaust air side, and (c) the laser beam sensor from 37 to 43 min in Fig. 9

Figure 11

Calculated average frost thickness vs time

Figure 12

Schematic of frost distribution on the cold matrix surfaces for one flow channel during the first cycle of frost growth

Figure 13

Schematic of frost distribution (a) immediately after one frost fracture and (b) just before the next frost fracture

Figure 14

Simulated results of (a) frost thickness, (b) frost density, and (c) frost-air interface temperatures for 300 s of frost grow period at a distance x=50mm for T1=−40°C,ϕ1=50%,g1=2.41kg∕(sm2),T3=20°C,ϕ3=40%,g3=2.41kg∕(sm2), and wheel speed is ω=20rpm

Figure 15

Simulated results of (a) frost thickness; (b) frost density; and (c) frost–air interface temperatures variation with time for a typical two rotations for conditions as in Fig. 1

Figure 16

Simulated results of (a) frost thickness; (b) frost density; and (c) frost–air interface temperatures for a period of 300 s of frost growth after frost fracture at 40 min at a distance x=50mm for flow conditions as in Fig. 1

Figure 17

Simulated averaged frost accumulation rate for 300 s of frost grow period within the regenerative wheel for flow conditions as in Figure 1

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