Investigation of the flow of Newtonian fluids in circular horizontal tubes at low inlet pressures

The important aspects of increasing hydrodynamic efficiency, improving performance and technical characteristics of various heat and mass transfer equipment, as well as ensuring the required regime and flow conditions of fluids with different viscosities have been analyzed. Such heat transfer equipment includes radiators that contain tubes for circulating heat transfer medium. When solving related issues, in addition to studying the flow characteristics of various fluids in circular horizontal tubes (capillaries), it is necessary to determine the conditions, under which the flow of fluid inside capillaries and circular tubes of small diameter is laminar, so it can be described by the Poiseuille equation. The experimental data on determining water flow rates in horizontal circular tubes of various diameters are presented. The dependence of the volumetric flow rate on the pressure drop has been determined. It was shown that the basic parameters that determine the flow characteristics of fluids in horizontal tubes are the tube radius and fluid dynamic viscosity. A flow of distilled water in tubes with diameters of 0.95, 1.6 and 2.0 mm was analyzed at a gauge pressure ranging from 0.266 to 4.000 kPa. It was found that when using a 0.95 mm diameter tube, the dependence of the volumetric flow rate on the gauge pressure remains linear in the entire analyzed range of pressures. An increase in the tube radius increases the likelihood of flow velocity fluctuations and the appearance of a radial velocity component (i.e., the occurrence of the elements of a turbulent fluid flow structure). The water flow regime in tubes with diameters of 1.6 and 2.0 mm deviates from the laminar at pressures exceeding 1.3 and 1.0 kPa, respectively. The dependence of the volumetric flow rate on pressure for a 40% aqueous solution of calcium chloride, as well as transformer, transmission, and engine oils with dynamic viscosities ranging from 0.002 to 0.182 Pa · s remains linear up to the tube diameters of 5–6 mm. The results of studying fluids with different viscosities are presented in the form of a nomogram illustrating the ratio of the tube radius raised to the fourth power to the fluid viscosity as a function of the tube radius. By analyzing this relationship, it becomes possible to predict the flow characteristics of the studied fluid at given tube radius and dynamic viscosity values. The obtained results can be used when designing and operating various heat transfer devices, such as radiators with tubes for circulating a heat transfer medium.