Heat and Mass Transfer
Two long parallel surfaces each of emissivity 0.7 are maintained at different temperatures and accordingly have radiation heat exchange between them. It is desired to reduce 75% of the radiant heat transfer by inserting thin parallel shields of emissivity 1 on both sides. The number of shields should be

Two
Three
Four
One

ANSWER DOWNLOAD EXAMIANS APP

Heat and Mass Transfer
The logarithmic mean temperature difference (tm) is given by (where Δt1 and Δt2 are temperature differences between the hot and cold fluids at entrance and exit)

tm = tm = (Δt1 - Δt2) loge (Δt1/Δt2)
tm = (Δt1 - Δt2)/ loge (Δt1/Δt2)
tm = loge (Δt1/Δt2)/ (Δt1 - Δt2)
tm = loge (Δt1 - Δt2)/ Δt1/Δt2

ANSWER DOWNLOAD EXAMIANS APP