稳流过程 | Steady Flow Processes

控制体积

Control volume (open system) – A system that involves mass flow across their boundaries

稳流系统

Steady flow systems – control volume with steady mass flow

质量守恒 | Conservation of mass

m_in and m_out -> mass flow rate

m_cv -> control volume mass change rate

稳流过程：压缩和非压缩过程 | Steady flow process – Compressible / Incompressible Flows

对于稳流过程，有公式：

where v is velocity.

对于存在压缩的情况,

对于不存在压缩的情况,

流体功 | Flow work

Work done by the flow push.

F = pA

W_flow = FL = pAL = pV

where p is pressure and V is volume.

流动流体的总能量 | The total energy of a flowing fluid

焓 | Enthalpy

用焓代入上面的公式，得到：

稳流系统 | Steady flow systems

In engineering devices, such as turbines, compressors, andnozzles operate for long periods of time under the sameconditions, once the transition start-up period is completed andsteady operation is established, they are classified as steady flow devices, which means the process is a steady flow process.

在一段时间内

对于不存在能量存储的情况，E_in = E_out

所以，

管道与管道流动 | Pipe and duct flow

E_in - E_out = E_system = 0

喷嘴和扩散器 | Nozzles and diffuser

A nozzle is a device that increases the velocity of a fluid at the expense of pressure.

A diffuser is a device that increases the pressure of a fluid by slowing it down.

节流阀 | Throttling valves

节流阀可以被看做是绝热的(adiabatic)因此节流阀中可以看做不存在焓变。

h_in = h_out

流体能 pv (pressure * volume) 增加

p_inv_in < p_outv_out

内能降低

u_in > u_out

混合室 | Mixing chambers

Mixing two streams of fluids.

热交换器 | Heat exchangers

压缩器 | Compressors

伯努利方程 | Bernoulli’s equation

In most flows of liquids, and of gases at low Mach number

v -> 单位体积 specific volume (m³/kg) ;

V -> 速度 velocity (m/s)

例题

The electric heating systems used in a house consist of a simple duct with a resistance heater. Air is heated as it flows over resistance wires. Consider a 15 kW electric heating system. Air enters the heating section at 100 kPa and 17°C with a volume flow rate of 150 m³/min. If heat is lost from the air in the duct to the surroundings at a rate of 200 W,

determine the exit temperature of air. c_p = 1.005 kJ/kg·°C at room temperature.

由公式流体的总能量 ，系统能量变化 ，

对于本题，

由于压强 p 和体积 v 不变，，

由公式 ，。

则最终的温度

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