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Air Jet Experimen t喷气实验
Abstract摘要
A plate with a spring gauge is placed at the Pitot tube probe and there is an entry called orifice and exit called nozzle for the air generated by a fan. 皮托管探头被放置在A板弹簧压力表上，其中有一个项目由风扇产生的空气作用到喷嘴口和出口上的一个项目。The fan gives air flow which makes the pressure on the plate on the opposite direction of the gravity as a force. The force on the plate can be measured in this experiment or calculated from the theory. Additionally the nozzle forms a jet boundary which is variable by the height of the plate. It was observed that the nozzle gives larger diameter of the boundary as the height of the plate increases. The force calculated in the theoretical work is larger than observed in the experiment.在实验中观察到，理论中计算的工作要大于实际的作用力
The experimental problem and theory实验题和理论
The first experiment is to study the force of a plate impinged by an air jet. The second experiment is to study the formation of a round air jet by a nozzle, tested by a plate. 第一个实验是研究由空气射流的冲击力。第二个实验是研究形成一个圆形空气射流喷嘴的力度。
The force applied on the fluid with a certain length of time period has the result to change the momentum of the fluid. In other words, the force has the effect as the velocity change of the fluid at its mass flow rate. http://www.ukassignment.org/dxessay/
Methodology方法论
The first experiment is to measure the force on the round plate by jet below, for different distances.
第一个实验是测量的圆盘上通过喷射的力，以供不同的距离。如下：
The second experiment is to measure the effective area of a jet. In additional to the height of the plate is a variable, the radial distance is another one so that at large distance it will ultimately gives zero force.
The plate has a spring gauge so that the force on it can be measured. 该板具有一个弹簧计可以测量的力，这样就可以了。The plate can be placed above the nozzle and adjusted to different heights and redial distances. The pitot tube has a probe which connects the manometer. Fan gives air jet to the nozzle.
In experiments results are collected in trials which are compared with theoretical calculations.
实验的结果，通过在试验与理论计算的数据收集来进行比较。
Results and Discussion结果与讨论
For the first experiment plate heights from 0 to 90 cm are measured with step size 10 cm. From 0 to 90 cm the force measured are 1200g, 1080g, 1060g and so forth, respectively. In other words from 20 cm up the force remained in 1060g. The fan gives a pressure as 25mm fluid level increase was observed.风扇给人以25mm的压力上升着。
Pressure
P = 784* 9.8 * 0.025*sin15
Velocity at the orifice
v(d) = sqrt(2P/Rho(air))
Exit velocity
v(e) = v(d) * (70/25)^2 = 71.07
mass flow rate
Rate(m) = C(d)rho(air)v(e)A(e) = 0.6*1.21*v(e)*A(e)= 0.0253 kg/s
Air density used = 1.21 kg/m^3
Force on the plate can be derived as
F = rate(m)*v(e) = C(d)*rho(air)*v(e)^2*A(e) = 1.798 kg
This value is larger than the experiment observed, which is about 1.08, 1.06 kg.
For the second experiment heights from 0 to 80 cm with step size 10 cm are measured with each height has 8 trials in different distances from center to outskirt. The measured fluid levels in manometer give that for closer distance to the nozzle, the central pressure is larger than further distance to the nozzle (i.e. lower position vs. higher position). On the other hand the boundary of the nozzle is increased as the plate was moved to higher (further) position. In other words, to develop nozzle area it needs a certain distance as the basis. 换句话说，开发喷嘴区域，它需要有一定的距离为基础。
In experiment 2, the jet boundary can be approximated by average on the weighted sum of force at each height; weight is taken as the redial distance, hence It was observed that the nozzle gives larger diameter of the boundary as the height of the plate increases. The force calculated in the theoretical work is larger than observed in the experiment. 据观察，喷嘴会提供较大直径的边界作为板的增加的高度。理论计算的工作力大于在实验中观察到的力。 |
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