Flow maldistribution in heat exchanger tubes can significantly affect its performance. In this work, 16 tubes are connected between the inlet and the exit headers forming the heat exchanger. The feed nozzle is connected to the inlet header, and its connection point can be altered. The influences of inlet flow Reynolds number, nozzle diameter, number of nozzles and nozzle location on the flow maldistribution are experimentally investigated. Water is chosen to be the working fluid inside the heat exchanger set of tubes. At lower flow rates, the results showed that the flow Reynolds number has a significant effect on the flow maldistribution inside the heat exchanger set of tubes; however, at higher flow rates, this effect was insignificant. Locating the nozzle at the center of the inlet header resulted in about 25–30% reduction in the standard deviation (STD) of the flow rate inside the tubes. Increasing the number of inlet nozzles resulted in an insignificant effect on the flow maldistribution. Increasing the nozzle diameter resulted in increased STD of the flow rate distribution among the tubes and pressure drop across the tubes at the considered heat exchanger geometry and water flow rate.

This study aims to investigate experimentally the natural convection flow over an array of discrete surfaces mounted on a flat vertical substrate. The investigation covers the case where the discrete surfaces/substrate unit forms a wall of a closed cavity. Experimental measurements of the flow and thermal fields were conducted. The experimental study is aimed at investigating the influence of the Rayleigh number on the velocity and thermal fields inside the cavity. Two cases of heat source distribution were considered: uniform heat source distribution and non-uniform heat source distribution. Measurements of vertical and horizontal velocity components were obtained for three heating values: 10, 15, and 20 W corresponding to Rayleigh number values of Ra = 9.06 × 109, Ra 1.19 × 1010, and Ra 1.41 × 1010. For each case, the velocity measurements were conducted at three different vertical locations, located at the middle of each of the three heaters. Velocity measurements were also obtained for non-uniform heater distribution at Ra 9.53 × 109. The energy loss was found to be about 10% of the power input in each case. Empirical correlations were obtained for the data for uniformly distributed heaters.

Tube overheating may cause tube failure resulting in an unscheduled boiler shutdown that may interrupt plant operation. The impact of this problem is not only due to the cost of replacing defective parts but also due to the frequent need of system shutdown and the possible imminent safety hazards. This paper provides an investigation of the influence of rapid rise in steam flow rate (swing rate) on the thermal and flow characteristics of the riser tubes in natural circulation water tube boilers. A thermal model for the prediction of possible tube overheating was developed. The developed model incorporates a nonlinear state space dynamic model which captures the important physical interactions of the main variables of steam generation in drum boilers. The system under consideration includes the drum, the riser, and downcomer as its major components. A numerical scheme for the solution of the governing differential equations was established. The dynamic response of the system's state variables due to rapid rises in steam flow rate was investigated. The results show that the rapid rise in the steam flow rate results in decrease in the pressure and an initial increase in the steam quality which is followed by a decrease in the steam quality. The riser temperature increases partly due to the increase in the steam temperature and partly due to the dynamic influence resulting from a decrease in the heat transfer coefficient. The present calculations of the water level in the drum provide good comparison with those in the literature.

A thermal model for the prediction of possible tube overheating was developed. The model incorporates a nonlinear state space dynamic model that captures the important physical interactions of the main variables of steam generation in naturally circulated water tube drum boilers. This paper provides an investigation of the dynamic effects of rapid rise in fuel flow rate (heat input) on the thermal and flow characteristics of the riser tubes in natural circulation water tube boilers. The system under consideration includes the drum, riser and downcomer as its major components. The dynamic response of the system's state variables due to rapid rises in fuel flow rates was investigated.The results show that the sudden rise in the firing rate is followed by an increase in the steam quality, which is accompanied by a decrease in the circulation rate as a result of increase in the pressure. The riser temperature increases slightly above the saturation temperature due to the increase in the steam temperature and due to the dynamic influence resulting from sudden increase in the heat flux. The present calculations of the water level in the drum provide good comparison with those in the literature. Copyright (C) 2009 John Wiley & Sons, Ltd.