# The purpose of this problem is to investigate the influence of flow gross maldistribution on an…

The purpose of this problem is to investigate the influence

of flow gross maldistribution on an air-cooled air-conditioning condenser with

round tubes and flat fins (Fig. P7.18). Because of the specific packaging

arrangement, the condensers face area is blocked by 50%, as shown in Fig.

P7.18b. Assume that the total airflow rate over the partially blocked condenser

of Fig. P7.18b is the same as the airflow rate over the unblocked condenser of

Fig. P7.18a. The Nusselt number and friction factor correlations for the

airside surface are as follows:

Determine the following, mentioning

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The purpose of this problem is to investigate the influence

of flow gross maldistribution on an air-cooled air-conditioning condenser with

round tubes and flat fins (Fig. P7.18). Because of the specific packaging

arrangement, the condensers face area is blocked by 50%, as shown in Fig.

P7.18b. Assume that the total airflow rate over the partially blocked condenser

of Fig. P7.18b is the same as the airflow rate over the unblocked condenser of

Fig. P7.18a. The Nusselt number and friction factor correlations for the

airside surface are as follows:

Determine the following, mentioning clearly any additional

assumptions that you may make for the solution during each step.

(a) Percentage increase or decrease in the air-side heat

transfer coefficient due to blockage of the face area.

(b) Percentage increase or decrease in the air-side pressure

drop.

(c) Percentage increase or decrease in ohA on the air

side. Here assume that o is sufficiently high that it changes negligibly

with the change in h.

(d) The total thermal resistance for the unblocked condenser

in terms of the airside thermal resistance assuming negligible fouling and wall

resistances. For this case, the ratio ohA on the air side to hA

on the refrigerant side is 1

3. Note that there are no fins on the refrigerant side.

Hint: Use Eq. (3.24).

(e) The total thermal resistance of the partially blocked

condenser in terms of its air-side thermal resistance, again neglecting fouling

and wall resistances. Note that the refrigerant-side heat transfer coefficient

for unblocked and partially blocked condensers remains the same (since the

refrigerant passages are not blocked); only its heat transfer surface area is

affected.

(f ) The NTU of the partially blocked condenser, assuming

the NTU of the unblocked condenser to be 0.5 and UAs known from parts (d) and

(e).

(g) Finally, determine the reduction in the heat transfer

rate of the condenser assuming that C* = 0 and the same inlet temperature

difference Tmax.

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