بررسي اثرات غيرماندگاري افت اصطكاك و شدت جريان گذرا در روش هاي نشت يابي با استفاده از روابط انرژي يك بعدي

INVESTIGATION OF UNSTEADY EFFECTS OF FRICTION LOSS AND TRANSIENT FLOW USING 1D ENERGY RELATIONS

تحليل معكوس جريان گذرا به‌منزله‌ي يكي از روش هاي كارا جهت شناسايي نشت در خطوط لوله شناخته مي شود كه در سال هاي اخير مورد توجه بسياري از پژوهشگران بوده است. در اين نوشتار به‌منظور تعيين شرايط مناسب جهت پياده-سازي روش هاي نشت يابي بر پايه‌ي مدل سازي جريان گذرا، به تحقيق درخصوص اثر مدت زمان بستن شير در شرايط جريان آرام و آشفته با عدد رينولدز پايين و اثرات غيرماندگاري افت اصطكاك پرداخته شده است. نتايج نشان مي دهد كه با استفاده از اعداد رينولدز پايين تر و مدت زمان هاي طولاني تر بستن شير مي توان به نتايج مناسبي در روند نشت يابي رسيد. بدين ترتيب علاوه بر آنكه مي توان اعتمادپذيري نتايج را تا حد قابل قبولي ارتقا داد، از خسارت هاي ناشي از نوسانات شديد كه در نتيجه‌ي جريانات گذراي سريع ايجاد مي شود دوري جست.

Leaks in pipes and water distribution systems might occur for various reasons, such as poor quality of pipe material, errors in operation and maintenance, corrosion, and internal or external high pressure. As a result, loss of water, environmental problems, extra energy consumption and unnecessary pump capacity could be observed. To avoid all these consequences, it is essential to provide advanced monitoring methods to observe resulting problems and their extent, and then to find and implement solutions for water transportation and distribution systems to reduce loss and increase energy efficiency. Several leak detection methods have been introduced up to now, such as inverse transient analysis, to overcome these problems. Application of the energy concept to fluid transients in closed conduits leads to an alternative description of unsteady flow behavior. In this interpretation, a transient in a pipe system can be viewed as a sequence of energy transformations, which moves the system from some initial hydraulic conditions to some other final states. During this conversion, mechanical energy is dissipated and work is done on the fluid. When the rate of flow in a closed conduit is changed, large-scale conversions of mechanical energy often occur, particularly if the pipeline is carrying water or any other slightly compressible liquid. It is obvious that the side-flow volume flux has a marked effect on the unsteady frictional dissipation component of the fluid. Mathematical expressions describing these transient energy transformations are first derived from main principles and then the governing continuity and momentum equations are mathematically manipulated to provide the final set. Those various terms, which must be accounted for in the analysis, include the energy dissipated by fluid friction, the work done at the upstream and downstream ends of the pipe and the kinetic energy carried into and out of the conduit. In this paper, the leakage term is added into continuity and momentum equations and, therefore, the energy relation for the dissipated energy due to leakage could be derived. Then, in order to determine the appropriate condition for modeling the detection methods, based on transient flow simulation, the effects of the time of valve closing in laminar and turbulent flow, with low Reynolds numbers, have been investigated. The results show that by using lower Reynolds numbers and longer durations of valve closing, appropriate results could be obtained for leak detection. In this manner, not only is the reliability of the results increased, but those sharp fluctuation indemnities caused by the fast transient flow are also avoided.