Takaki OHKOUCHI Hiroyuki OSAKABE Toshihide NINAGAWA Kiyoshi KAWAGUCHI

Transcription

1 The Heat Transfer and Pressure Drop Characteristics of the Heat Exchanger for Recovering Latent Heat (The Heat Transfer and Pressure Drop Characteristics of the Heat Exchanger with Wing Fin) Takaki OHKOUCHI Hiroyuki OSAKABE Toshihide NINAGAWA Kiyoshi KAWAGUCHI In recent years the requirement for reduction of energy consumption has been increasing to solve the problems created by global warming and the shortage of petroleum resources. The latent heat recovery type heat exchanger is one of the effective methods of improving thermal efficiency by recovering latent heat. This paper describs how the heat transfer and pressure drop characteristics of the latent heat recovery type heat exchanger having a wing fin (fin pitch: 4mm, fin length: 65mm) were clarified by measuring the exchange heat quantity, the pressure drop of heat exchanger, and the heat transfer coefficient between the outer fin surface and gas, and also the effects of the behavior of condensate on the fins on heat transfer and pressure drop characteristics were clarified. Furthermore the equations for predicting the heat transfer coefficient and pressure drop which are necessary in the design of the heat exchanger were proposed. Key words : Heat exchanger, Heat transfer, Pressure drop, Latent heat De f f p f p G

2 h g H H K i Nu Q w Re t f T i U f U g X z P P i P core ρ i σ 65 Gas Outlet of cooling water Fin Cooling water Inlet of cooling water 4 Fig. Structure of heat exchanger Cooling water Path of gas Table Fin specification Heat exchanger Fin shape Fin length Fin pitch EX. Straight fin 65mm 4mm EX.4 Wing fin 65mm 4mm Fin.8 4 Fig. Shape of straigt fin Fin.8 Wing 4 Fig. 3 Shape of wing fin

3 Chamber (5 5mm) Electric heaters for air Orifice for measuring flow rate Vaporized water Liquid water Thermocouples for measuring inlet gas temperature Duct Nozzle Humidifier Psychrometer Test section (4 mm) Orifice Control valve Pump Tank for returning cooling water Blower for supplying air to test section P Thermocouples for measuring cooling water temperature Pressure loss of heat exchanger by measuring differential pressure Thermocouples for measuring outlet gas temperature Electric heaters for water Measuring cylinder to measure quantity of condensed water Tank for supplying cooling water to test section Fig. 4 Experimental apparatus

4 Quantity of heat Qw (kj/s) H=65mm, fp=4.mm T=ºC t=ºc, Ww=6L/min, Dry, X=.3g/g, Dry, X=.3g/g Gas velocity Ug (m/s) Fig. 5 Relationship between quantity of heat and gas velocity in cases of straight fin and wing fin Left H=65mm, fp=4.mm T =ºC Ug =.m/s t =ºC W =6L/min Center H=65mm, fp=4.mm. T =ºC Ug =.m/s. t =ºC.3 W =6L/min Right H=65mm, fp=4.mm T =ºC Ug =.m/s t =ºC W =6L/min Fig. 6 Distribution of gas temperature in fin (Dry)

5 Left H=65mm, fp =4.mm T =ºC Ug =.m/s t =ºC Ww =6L/min Center H=65mm, fp =4.mm. T =ºC Ug =.m/s. t =ºC.3 Ww =6L/min Right H=65mm, fp =4.mm T =ºC Ug =.m/s t =ºC Ww =6L/min Fig. 7 Distribution of temperature of fin surface (Dry) Pressure loss P (Pa) H=65mm, fp=4.mm T=ºC t=ºc, Ww=6L/min, Dry, X=.3g/g, Dry, X=.3g/g Gas velocity Ug (m/s) Quantity of heat Qw (kj/s) H=65mm, fp=4.mm T=ºC t=ºc, Ww =6L/min X =.3g/g, Latent heat, Latent heat Gas velocity Ug (m/s) Fig. 8 Relationship between pressure drop and gas velocity in cases of straight fin and wing fin Fig. 9 Relationship between latent quantity and gas velocity in fin

6 Left Center Right H =65mm, fp=4.mm T =ºC Ug =.m/s t t =ºC Ww=6L/min X =.3g/g H =65mm, fp=4.mm T =ºC Ug =.m/s t t =ºC Ww=6L/min X =.3g/g Straight fin Wing fin H =65mm, fp=4.mm T =ºC Ug =.m/s t =ºC Ww=6L/min X =.3g/g Straight fin Wing fin Fig. Distribution of gas temperature in fin (Wet) Left Center Right H =65mm, fp=4.mm T =ºC Ug =.m/s t =ºC Ww =6L/min X =.3g/g H =65mm, fp=4.mm T =ºC Ug =.m/s t =ºC Ww =6L/min X =.3g/g H =65mm, fp=4.mm T =ºC Ug =.m/s t =ºC Ww =6L/min X =.3g/g Fig. Distribution of temperature of fin surface (Wet)

7 Fig. Behavior of condensate in straight fin Fig. 3 Behavior of condensate in wing fin Gas flow Gas flow Gas flow Small water droplets Large water droplets Water film Fig. 4 Micro view of behavior of condensate in straight fin

8 Gas flow Gas flow Gas flow Gas flow Wing Water film Small water droplets Large water droplets Fig. 5 Micro view of behavior of condensate in wing fin Nusselt number Nu 5 5 Wing fin H=65mm, f p =4.mm T = t =, W w =6L/min X=dry X=.3g/g 3 Reynolds number Re Gas flow Nu = Nu = Inlet Re Pr Re Pr Core region Outlet p core p p p Pressure drop due to droplet Pressure drop due to wing Fig. 7 Pressure drop model of heat exchanger 3 Re 6 Fig. 6 Relationship between Nusselt number and Reynolds number

9 p = p + pcore Pressure loss P (Pa) p G G ( σ ) + K ( σ ) G G p p = c K e ρ ρ ρ ρ p core G f Y n K = + ρ D av e Dry Wet (X=.3g/g) G w ρav Gas velosity Ug (m/s) Fig. 8 Relationship between pressure drop and gas velocity in fin K f G + ρ av

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