课程名称︰热传学 课程性质︰ 课程教师︰马小康 开课学院：工学院 开课系所︰机械系 考试日期（年月日）︰2010/12/14 考试时限（分钟）：100 是否需发放奖励金：是 （如未明确表示，则不予发放） 试题 : (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)── = ( ── ) (此处的d为偏微分,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 (正负号看座标轴方向) ( 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为fin的长度) ▏／￣￣￣￣￣▏／ (A为fin之间的间隔) ▏￣￣￣￣￣￣▏ Number of fins : N ￣￣￣￣￣￣￣ Ub = Ubulk = 空气流入的速度 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 --

※ 发信站: 批踢踢实业坊(ptt.cc) ◆ From: 123.204.69.54 ※ 编辑: hanson0508 来自: 123.204.69.54 (02/06 15:37)