Home | IFS Content | History | Purchase Internal Flow Systems Experimental Measurement of Loss Coefficients Part 1 - Why Guidance on Experimental Projects is Required
1.1 Introduction
The class 1 and 2 loss coefficients in Internal Flow Systems were generated by individuals and teams who built up experimental capabilities and expertise over several years. Present day research environments do not allow for a gradual build up of capabilities, making it essential that experimenters have guidance on the tens of geometric, fluid dynamic, measurement and analysis parameters involved in determining loss coefficients.
Important aspects of experimental projects on internal flows have been done many times before. In industry repetitive aspects of design are codified so that projects are carried out to specified standards. This allows designers and engineers to maximise creative input on aspects unique to their project. Writing a guide on experimental internal flow projects is not an attractive proposition because of the small potential user base and the difficulty of reaching potential users. Probably the best we can do is to use the World Wide Web to develop a checklist and procedures. Contributions for a checklist are welcome. Three recent substantial experimental programmes (1-3) that measured component loss coefficients illustrate why guidance is necessary.
- The literature reviews carried out by the three projects are poor. No critical assessment is made of the quality and relevance of the loss coefficient data in the references quoted. References in design guides were not followed up to access information that would have assisted in planning and executing the projects.
- One project on bends has a negative outcome because the loss coefficients cannot be related to known reliable bend loss coefficient data
- One project only measured part of the pressure loss due to components and the percentage of pressure loss varied between components. Corrections can be made to the measured loss coefficients to make them compatible with loss coefficients in Internal Flow System but doubts will remain as to their accuracy
- Two of the projects involved extensive testing of commercial piping system components. Value was not added by relating measured loss coefficients to component and system geometric parameters
- The value of the three projects would have been greater if they had set out to determine correction factors to account for departures from reference geometries and test conditions.
Bends are the commonest component in piping systems, so it is appropriate to look at the state of the literature related to bends. This is followed by considering how references should be reviewed. A review of a reference is carried out in Part 2 to help in understanding what should be included in a project checklist in Part 3.
1.2 Bend Loss Coefficients
Over 200 references related to flow through bends were known and many reviewed in (4) prior to the BHRA research in the 1960s that provided the Class 1 bend loss coefficients in Internal Flow System. Since (4) was written many more papers on bends have been published. There are now numerous loss coefficients for bends in the literature but most are not for defined bend geometries, measured in defined system arrangements in a defined manner.
Experiments at BHRA on bends in the 1960s generated basic loss coefficients that provide reference loss coefficients for experiments. Basic loss coefficients apply to bends that meet all of the following:
1. Accurately made.
2. Bend geometry described by a radius ratio, a deflection angle and a cross sectional shape.
3. Hydraulically smooth internal surfaces.
4. Long hydraulically smooth inlet pipe or passage to provide reasonably developed flow at inlet to a bend.
5. Long hydraulically smooth outlet pipe or passage to re-establish a developed flow friction gradient.
6. Flow at a Reynolds number of 10E6.
7. Pressure loss over and above friction loss in developed flow attributed to a bend.
Having determined basic loss coefficients we need means of applying them to industrial flow systems. Correction factors are used that account for departures from:
1. Basic component geometric parameters including surface roughness.
2. Basic installation parameters that affect inlet and outlet conditions such as pipe lengths, surface roughness and interactions with other components.
3. Reference Reynolds number.
The aim is to change from endlessly measuring loss coefficients to a situation of determining correction factors to apply to basic loss coefficients to account for departures from reference conditions. Taking the correction factor approach we gain greater insight into internal flows and, very importantly, we can provide designers and engineers with easier to use and more comprehensive information. 1.3. Reviewing a References
The literature review is where project credibility starts – is the project grounded in the real world or is it in a project world of its own. Grounding in the real world requires reference points from which to move into the unknown. If a project does not have reference points it is adrift and it is highly likely that what is measured will not or cannot be related to the real world.
An investigative approach is particularly important when reviewing prior work. If you reference the results of others in your project you need to establish to your satisfaction that:
- You know the geometries of the components tested. Unless a component can be described by two or three non-dimensional geometric ratios, a drawing is needed to know what was tested. Few projects provide such information. How will you justify referencing a project if you do not know what was tested?
- You know the arrangement of the system in which a component was tested. Did the system provide for defined conditions at inlet to a component and for losses caused downstream of a component? If not, is sufficient information given for you to make informed adjustments to loss coefficients so they are compatible with your coefficients?
- The write up of a project tells you that the researcher/s took an investigative approach to the references they used, or did not use? If you find a project used others questionable results, without comment on their validity, what does this tell you?
- The write up on a project indicates to you that the researcher/s understood internal flow behaviour. You need to comment on any deficiencies in the references you quote; otherwise your understanding of internal flows will be questioned
You should not accept uncritically loss coefficients in published work. Informed scepticism is required given the frequency of even quite basic errors. The fact that a paper has been through a journal's review process and has been referenced numerous times is no guarantee that the loss coefficients it contains are for known component geometries, tested under defined conditions. You should think of the literature review for a project as being the act of carrying out due diligence. A recent project to measure loss coefficients is reviewed in Part 2 to illustrate the importance of taking an investigative approach to prior work.
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