Friction Losses Lab Report - Fluids

Topics: Fluid dynamics, Reynolds number, Viscosity Pages: 18 (1692 words) Published: February 2, 2014


Friction Losses

Abstract—The purpose of the experiment is to study the differences of roughness, valves and geometries of pipe and how they influence friction losses.

Introduction
Friction loss is the loss of energy or “head” that occurs in pipe flow due to viscous effects generated by the surface of the pipe. This energy drop is dependent on the wall shear stress (τ) between the fluid and pipe surface. The shear stress of a flow is also dependent on whether the flow is turbulent or laminar. For turbulent flow, the pressure drop is dependent on the roughness of the surface, while in laminar flow, the roughness effects of the wall are negligible. This is due to the fact that in turbulent flow, a thin viscous layer is formed near the pipe surface which causes a loss in energy, while in laminar flow, this viscous layer is non-existent. Causes of friction loss can include the movement of fluid molecules against one another, or against the inside surface of the pipe and bends, kinks or sharp turns in hose or piping. This experiment allows us to investigate different scenarios of piping, particularly in roughness, geometry and valves. With the many circuits of flow to chose from the student can combine different variations of each to see how the flow responds.

Governing Equations
Basic Head Loss for Strait Pipes
Head loss can be expressed as a function of friction factor: (eq 1)

Where hf is head loss, L is length, D is diameter of pipe, V is the velocity of the flow and g is gravity.

Flow Rate
The flow rate in a pipe can also be calculated using

(eq 2)

In this case u is used for velocity, Q as volume flow rate and A, cross sectional area of the pipe.

Reynold’s Number
In fluids, the Reynolds number (Re) is a dimensionless number which gives a measure of the ratio of inertial forces to viscous forces. The Reynold’s Number is also used to classify laminar and turbulent flow values. When working with pipes that have different diameters and different flow rates it is easy to make comparisons using Reynold’s Number. The calculation of the Reynold’s number is as follows: (eq 3)

Re denotes the Reynold’s number, u is the average velocity of the fluid, d is diameter of the pipe and v is the kinematic velocity of the fluid, which for water at 20C we will use the value 1.004x10-6.

Blasius Friction Factor
The Blasius equation defines only the lower boundary of the friction factor. It can be found using the following:

(eq 4)
Loss in Strait Pipes
The loss from strait pipes can be characterized by a length found by:

(eq 5)

Head Loss in Bent Pipes
These types of losses are represented with a loss factor, k. The equation for head loss in bent pipes then becomes

(eq 6)

Here the k value can either be kL or kB. kL denotes total head loss around the bend where kB is for losses that are due to the bend geometry (likewise with hL and hB denoting total losses and losses from pipe geometry).

Geometry

Figure 1: The H408 Fluid Friction Apparatus

Figure 2: Layout of the Fluid Friction Apparatus

In the apparatus shown in Figure 1, there are three main flow circuits which are color coded. The circuits are shown in Figure 2 with each of their labeled components. Each of these circuits have a control valve and selections of pipes and fittings, a list of which can be seen in Table 1. There are pressure tappings that are numbered and fitted at the important points of interest to measure the pressure change along each pipe section. To measure the pressure change across a pipe section there is a free standing piezometer connected. No included with the TecQuipment but used in our lab, was a hydraulic bench for the water supply and flow measurement; it can be seen in Figure 3. This tank was used to time the flow rate supplied to the circuits.

Table 1: List of pipe fittings...


References: H408 Friction Fluid Apparatus User Guide. TecQuipment LTD 2008.
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