## Heat Transfer Miscellaneous

#### Heat-Transfer

1. Lumped heat transfer analysis of a solid object suddenly exposed to a fluid medium at a different temp is valid when

1. For lumed system analysis, Bi < 0.1

##### Correct Option: A

For lumed system analysis, Bi < 0.1

1. When the fluid velocity is doubled the thermal time constant of a thermometer used of measuring the fluid temperature reduces by a factor of 2 (T/F)

1. It is not necessary that h would become a half if velocity becomes double.

##### Correct Option: D

It is not necessary that h would become a half if velocity becomes double.

1. Biot number signifies

1. Biot number = Rconduction/Rconvective

##### Correct Option: D

Biot number = Rconduction/Rconvective

1. The heat transfer process between body and its ambient is governed by an Intemal Conductive Resistance (ICR) and an External Convective Resistance (ECR). The body can be considered to be a lumped heat capacity system is

1. Lumped heat capacity can be applied when Bi < 0.1
Bi hl/k, for desired condition i.e. Bi < 0
k > h, ICR is small

##### Correct Option: D

Lumped heat capacity can be applied when Bi < 0.1
Bi hl/k, for desired condition i.e. Bi < 0
k > h, ICR is small

1. In a concentric counter flow heat exchange, water flows through the inner tube at 25°C and leaves at 42°C. The engine oil enters at 100°C and flows in the annular flow passage. The exit temperature of the engine oil is 50°C. Mass flow rate of water and the engine oil are 1.5 kg/s and 1 kg/s, respectively. The specific heat of water and oil are 4178 J/kgK and 2130 J/kgK, respectively. The effectiveness of this heat exchange is

1. Tc,i = 25°C;
Tc,o = 42°C;
mc = 1.5 kg/s;
Cc = 4.178 kJ/kg.k
Th,i = 100°C;
Th,o = 50°C;
mh = 1 kg/s;
Ch = 2.130 kJ/kg.k
mcCc = 1.5 × 4.178
= 6.267 kW/°C = Cmax
mhCh = 1 × 2.130
= 2.130 kW/°C = Cmin

 effectiveness = ε = Ch(Th,i - Th,o) Cmin(Th,i - Tc,i)

 = 100 - 50 = 0.666 100 - 25

##### Correct Option: A

Tc,i = 25°C;
Tc,o = 42°C;
mc = 1.5 kg/s;
Cc = 4.178 kJ/kg.k
Th,i = 100°C;
Th,o = 50°C;
mh = 1 kg/s;
Ch = 2.130 kJ/kg.k
mcCc = 1.5 × 4.178
= 6.267 kW/°C = Cmax
mhCh = 1 × 2.130
= 2.130 kW/°C = Cmin

 effectiveness = ε = Ch(Th,i - Th,o) Cmin(Th,i - Tc,i)

 = 100 - 50 = 0.666 100 - 25