課程名稱︰熱傳學
課程性質︰
課程教師︰馬小康
開課學院：工學院
開課系所︰機械系
考試日期（年月日）︰2010/12/14
考試時限（分鐘）：100
是否需發放獎勵金：是
（如未明確表示，則不予發放）
試題 :
(20%)(1) A fluid with negligible viscosity flows between parallel plates. The
low viscosity allows the assumption that the fluid velocity throughout
the flow is a constant value V. The energy equation becomes
dT d^2T
(V/α)── = ( ── ) (此處的d為偏微分,partial)
dx dy^2
(15%)(a) Derive an expression for NuH for the case of qH<0, qs=-qH
( 5%)(b) For a thermally fully developed flow, sketch the temperature
profile across the flow for the following the following case,
where TH is the temperature of the upper plate and heat flow
is positive in the y-direction.
qH
────── TH
y
0└x ──────
qs
Case:qH<0, qs=-qH (正負號看座標軸方向)
( 7%)(2) A man plays a golf game and hits ball with Re=2.0*10^5, he tries
to improve his hit distance with a smaller ball. Is his decision
correct? Please explain the possible criterion. (Assume hit ball
with the same force under the same weather conditions.)
(10%)(3) For a laminar flow, the local Nusselt number at location x over
a flat plate is
Nux=0.332Re^(1/2) *Pr^(1/3) Pr>0.6
Please derive the average Nusselt number under the same conditions.
(15%)(4) A solar collector consists of a horizontal copper tube of outer
diameter 5 cm enclosed in a concentric thin glass tube of 9 cm
diameter. Water is heated as it flows through the tube, and the
annular space between the copper and glass tube is filled with air
at 1 atm pressure. During a clear day, the temperature of the tube
surface and the glass cover are measured to be 60℃ and 32℃, resp-
ectively. Determine the rate of heat loss from the collector by naural
convection per meter length fo the tube.
(The same problem with textbook 9-105)
(24%)(5) For an internal flow,
Given: B.C.'s T0(X+)=Te , at 0 < X+ < a
T0(X+)=Te+Ta , at a < X+ < 2a
T0(X+)=Te+Ta+Tb, at 2a < X+ < 3a
T0(X+)=Te , at 3a < X+ < 4a
Assume Te is the entrance fluid temperature and wall temperatur
distribution is as Figure.
Please find
(8%)(a) Mean fluid temperature, Tm(X+)
(8%)(b) Heat flux distribution at wall, q0"(X+)
(8%)(c) NuX
T0│ Te+Ta+Tb
│ ┌──┐
│ Te+Ta│Tb │
│ ┌──┘ │
│ │Ta │
Te└──┴──┴──┴──┴─ X+
0 a 2a 3a 4a
Tm│
│
│
│
│
Te└──┴──┴──┴──┴─ X+
0 a 2a 3a 4a
q0"│
│
│
0├──┴──┴──┴──┴─ X+
│ a 2a 3a 4a
│
│
NuX│
│
│
0├──┴──┴──┴──┴─ X+
│ a 2a 3a 4a
│
│
(24%)(6)
A plate fin exchanger surface can be considered as a series of flat plates as
shown below. Various alternatives have been proposed for increasing the
present value of the average heat transfer coefficient h0, and the overall
conductance, (h0A). Determine whether the ratios (h/h0) and (hA)/(h0A0)
increase, decrease, remain the same or are indeterminate when the indicated
changes are made.
Assume all dimensions are large compared to the boundary layer thickness.
├→ Ｗ ←┤ Side view:
▁▁▁▁▁▁▁ ┬
／ ／▏ ↓
／ ／ ▏ ┌─────────┐
／ ／ ▏ │ ─── │
／ ／ ▏ Ｈ │─── ───│↓
／▁▁▁▁▁▁／ ▏ │ L ─── │Ａ
▏ ▏ ▏ ↑ │─── ───│↑
▏／￣￣￣￣￣▏ ▏ ┴ │ ─── │
▏￣￣￣￣￣￣▏ ／ │─── ───│
▏╱￣￣￣￣￣▏ ／ └─────────┘
▏￣￣￣￣￣￣▏ ／ (L為fin的長度)
▏／￣￣￣￣￣▏／ (A為fin之間的間隔)
▏￣￣￣￣￣￣▏ Number of fins : N
￣￣￣￣￣￣￣ Ub = Ubulk = 空氣流入的速度
change (h/h0) (hA)/(h0A0)
Ub to 2Ub ─── ───
L to 2L ─── ───
L to 2L and N to 2N ─── ───
a to a/2 and N to 2N ─── ───
W to W/2 and H to H/2 ─── ───
Ub to 2Ub and L to 2L and N to N/2 ─── ───
*use S: Same or I: Increase or D: Decrease
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※ 編輯: hanson0508 來自: 123.204.69.54 (02/06 15:37)