Cross-flow Tube Heat Exchangers Research Articles

Crossflow polymeric hollow fiber heat exchanger: fiber tension effects on heat transfer and airside pressure drop

In various applications, a polymeric hollow fiber heat exchanger (PHFHE) is a competitive alternative to a conventional heat exchanger (HE). Standard empirical models for predicting the crossflow tube HE characteristics are defined for devices with rigid tubes with relatively large diameters compared to the polymeric hollow fibers with an outer diameter of around 1 mm. This study examines the impact of tension force on airside heat transfer rate and pressure drop in a crossflow PHFHE. The tension force influences the stiffness of the flexible polymeric fibers and their response to applied airflow. Two liquid–gas PHFHEs were designed and manufactured to ensure uniformity of the fibers' arrangement (inline and staggered). An experimental stand enabling the application of defined tension force in the range of 0–9000 N was designed, manufactured and placed into the calorimetric tunnel, where heat transfer rate and pressure drop measurement were performed with varying air velocity between 2 and 8 ms−1 (corresponding to Reynolds number of 240–970). Among our key findings was that the elongation of the fibers due to thermal expansion or stress relaxation has a considerable impact on the fibers' arrangement and resulting fluid flow. Moreover, the application of tension force yielded no substantial change in air pressure drop; however, it led to a notable enhancement in heat transfer rate. Specifically, under a maximal tension force of 9000 N, the heat transfer rate increased by around 11% compared to the unloaded state.

Analytical-numerical method for calculating cross-flow tube heat exchangers considering temperature-dependent fluid heat capacities

The paper presents a mathematical model of a steam reheater in a steam boiler installed in a power unit of 1000 MW. A new numerical-analytical method was used to determine the temperature distribution of steam, flue gas, and the reheater’s tube walls. The reheater is a two-pass cross-current heat exchanger. There are 44 panels of tubes on the width of the boiler. The steam flows parallel through 20 pipes in each pass. An analytical model of the superheater was developed with the assumption of the constant physical properties of steam and flue gas. The results of steam and flue gas temperature calculations obtained by the proposed method were compared with the exact method’s results. Very high accuracy of estimates was attained even if the whole superheater is divided into a few control volumes. The results of the first and second pass steam temperature calculations were compared using the P-NTU (Effectiveness - Number of Transfer Units) method. A perfect match between the steam temperature and the average temperature of the flue gas at the outlet of the reheater was found. The advantage of the developed method of superheaters modeling is the possibility to model heat exchangers with complex flow systems. The calculations were also carried out for temperature-dependent physical properties of steam and flue gas.

New analytical-numerical method for modelling of tube cross-flow heat exchangers with complex flow systems

A new method for thermal calculations of cross-flow tube heat exchangers was proposed. The heat exchanger is divided into finite volumes, inside which the fluid temperatures are given by analytical formulas. The advantage of the semi-analytical method developed is that the temperature inside the finite volume can be determined at any point, unlike the finite difference method widely used in exchanger heat modelling. Even when dividing the entire tube in one heat exchanger pass into a few finite volumes, e.g. into three segments, satisfactory results are obtained. By dividing the whole heat exchanger into a small number of finite volumes, the computer calculation time of heat exchangers with complicated tube arrangement is very short. The proposed method can be used to calculate heat exchangers with a complicated flow system in which the physical properties of fluids depend on temperature. Analytical and numerical models were developed for one and two pass finned tube heat exchangers to assess the accuracy of the proposed method. A two-pass car radiator mathematical model with two rows of tubes using the proposed method was developed. The results of the car radiator and steam reheater calculations are in good agreement with the analytical or experimental results.

New calculation method for tube cross-flow heat exchangers

A new method for thermal calculations of the cross-flow tube heat exchangers was proposed. The temperature of both fluids and the wall temperature are determined. The heat exchanger is divided into control volumes, in which outlet fluid temperatures are calculated by closed analytical formulas. Two examples of the application of the method for the calculation of two-pass cross-co-current and cross-countercurrent superheaters were presented. An exact analytical model was also developed for both superheaters to estimate the accuracy of the proposed method. The results of the superheater calculations using the developed method are in good agreement with the results obtained by the exact analytical models. The proposed method can be used to calculate heat exchangers with a complicated flow system in which the physical properties of fluids are temperature-dependent.

Experimental investigation on heat transfer and pressure drop of internal flow in corrugated tubes

The operating conditions of the pre-heater zone of an automotive vehicle waste heat recover heat exchanger occurs in, or close to, the transitional flow regime. For such an application the use of corrugated tubes in a cross flow tube heat exchanger is of particular interest. However, not much work has been done in the transitional flow regime in corrugated tubes where the internal flow changes from laminar to turbulent with heat transfer at high heat fluxes. Therefore, more data is needed. Hence, the main purpose of this work is the characterization of the heat transfer and pressure drop of internal flow in corrugated tubes. To this end an experimental setup was developed and validated for the study of a smooth tube and two corrugated tubes that have an internal diameter of 5.75 mm and a heating length of 0.38 m. The experiments were conducted in laminar, transitional and turbulent regime, with Reynolds numbers in the range from 429 to 6212, under both adiabatic and diabatic flow conditions. The heat flux imposed on the tube wall ranged from 5.5 kW/m2 to 21.1 kW/m2. The flow was hydrodynamically fully developed at test section inlet. The results revealed that the friction factor of the corrugated tubes presented a smoother transition than in the case of the smooth tube, being the diabatic and adiabatic friction factor for the corrugated tubes quite similar from laminar to turbulent regime. The corrugated tubes are more effective under transitional flow regime. The highest heat transfer augmentation occurs at Re ≈ 2000, specifically, the Nusselt number augmentation (Nuc/Nus) increases up to 4.7 for the corrugated tube with helical pitch p = 6 mm and up to 3.8 for the corrugated tube with p = 12 mm.

Selecting and Optimizing a Heat Exchanger for Automotive Vehicle Rankine Cycle Waste Heat Recovery Systems

Abstract This paper contributes for the selection and optimization of an adequate compact heat exchanger (HEX) for an automotive vehicle Rankine cycle Waste Heat Recovery Systems (RC-WHRS). Both tube type and plate type HEX were considered. Among the different HEX designs available, a robust cross-flow tube heat exchanger (CFT-HEX), with the working fluid circulating inside the tubes was selected. The selected HEX accomplishes with the specific pressure drops limits on both exhaust gas and working fluid side. The compactness of the CFT-HEX strongly depends on the optimization of the HEX pre-heater section, which mainly depends on the augmentation of the working fluid heat transfer coefficient. The present study data reveals that a transitional flow regime with a working fluid Reynolds number, Re > 3200, allows a significant increase of the Nusselt number (Nu > 20) as compared to laminar flow regime.

Mathematical Modeling of Cross-Flow Tube Heat Exchangers With a Complex Flow Arrangement

The general principles of mathematical modeling of heat transfer in cross-flow tube heat exchangers with complex flow arrangements that allow the simulation of multipass heat exchangers with many tube rows are presented. The finite-volume method is used to solve the system of differential equations for temperature of the both fluids and the tube wall with appropriate boundary conditions. A numerical model of a multipass steam superheater with 12 passes is presented. The convection and radiation heat transfer on the flue gas side are accounted for. In addition, the deposit layer is assumed to cover the outer surface of the tubes. Comparing the computed and measured steam temperature increase over the entire superheater allows for determining the thermal resistance of the deposits layer on the outer surface of the superheater. The developed modeling technique can especially be used for modeling tube heat exchangers when detailed information on the tube wall temperature distribution is needed.

Mathematical modelling of tube heat exchangers with complex flow arrangement

Mathematical modelling of tube heat exchangers with complex flow arrangement General principles of mathematical modelling of transient heat transfer in cross-flow tube heat exchangers with complex flow arrangements which allow a simulation of multipass heat exchangers with many tube rows are presented. First, a system of differential equations for the transient temperature of both fluids and the tube wall with appropriate boundary and initial conditions is formulated. Two methods for modelling heat exchangers are developed using the finite difference method and finite volume method. A numerical model of multipass steam superheater with twelve passes is presented. The calculation results are compared with the experimental data.