Search

• The desirable operating conditions to achieve high operational efficiency are obtained. • The performance of the IEC under different operating conditions is investigated. • The mathematical model is validated against published data. • The optimal dimensions that give good efficiency in climates with moderate humidity are highlighted. [Display omitted] This paper presents a study investigating and analyzing the effect of heat and mass exchanger (HMX) dimensions and the flow parameters on the indirect evaporative cooler (IEC) system's performance. For this purpose, a mathematical model based on heat and mass transfer concepts was developed, and the model was verified using experimental and simulation results from published papers. Simulations were performed based on different operating and structural conditions affecting the outlet air temperature, cooling capacity, coefficient of performance, and wet-bulb efficiency. Results showed that the optimal dimensions that give good efficiency in climates with moderate humidity, the length of the duct should be between 0.6 to 1.0 m , the width of the channel between 0.3 to 0.5 m , and the channel gap between 0.004 to 0.008 m. It has also been observed that the increasing working air velocity has a positive effect on cooling performance, and its velocity should not be less than 1 m / s. As for increasing the product air velocity, it reduces the heat exchange period, which is undesirable, so it should not exceed 1 m / s. The prescribed flow rate ratio between product and working air should be 0.3 to 0.5. This analysis provides desirable operating conditions to achieve high operational efficiency as well as optimal dimensions for designing such a cooler. [ABSTRACT FROM AUTHOR]

This paper presents a numerical study of heat and mass transfer in indirect evaporative air coolers with four air flow patterns: parallel-flow, counter-flow, cross-flow and regenerative. The numerical simulation was performed on the basis of original two-dimensional heat and mass transfer models (in the case of cross-flow heat exchanger the model was 3D). The mathematical models developed were validated against published experimental data presented in Appendix A . It was established, that heat and mass transfer processes in the wet channels of counter-flow, cross-flow and regenerative indirect evaporative coolers are characterized by creation of two particular heat and mass transfer zones. Detail analysis of the temperature and humidity ratio distributions and boundary conditions, characterizing the coupled heat and mass transfer process in each of these determined zones, revels the violation of the Lewis relation unity under a certain inlet and operation conditions. A theoretical method for estimating the Lewis factor was proposed. Using the developed models the thermal performances of the conventional designs of indirect evaporative air coolers were analyzed numerically and preferable climatic zones for considered heat exchanger were established. Four novel heat exchangers utilizing the advantages of the basic cycles heat exchangers were presented. [ABSTRACT FROM AUTHOR]

Abstract: The formation of particulate deposits on flue gas heat exchanger surfaces will reduce heat transfer efficiency, increase the instability of equipment operation and introduce a major uncertainty into the heat exchanger design. In this paper, a numerical model was developed to predict the flue-ash particle deposition rate by considering particles transport, sticking and rebound behaviors based on the software FLUENT, extended by user-defined functions (UDFs). The numerical model was applied to cross-flow tube bundle heat exchangers with a 6-row tube arrangement. The effects of six parameters (particle diameter, flow velocity, spanwise tube pitch, longitudinal tube pitch, tube geometry shape, and arrangement) on fouling rate, as well as on the heat transfer and hydrodynamics performance, were examined. It was found that particle deposits accumulated primarily in the flow stagnation region, recirculation region, the vortex separation and reattachment regions. Increasing particle diameter moved the deposition zones towards the windward side of tubes. Using both oval tubes and staggered arrangements can reduce the fouling rate. With the increase in longitudinal tube pitch, both the particulate deposit rate and the heat transfer performance increased. To account for fouling and heat transfer performance, a tube spacing value of 2 was recommended. [Copyright &y& Elsevier]

Abstract: In this paper a numerical model to analyze the thermal behaviour of vertical ground heat exchangers with a helical shaped pipe is presented. This type of configuration can be a suitable alternative to conventional ground heat exchangers, especially when the heating and cooling loads of the building are very low. The model describes the heat transfer problem by means of a network of interconnected thermal resistances and capacitances. Moreover, as the investigated ground heat exchanger is usually installed in shallow depth, the model takes into account the interaction between the ground and the ambient environment which affects the fluid heat carrier temperature into the heat exchanger and, as a consequence, the energy efficiency of the heat pump. After a sensitivity analysis on the mesh parameters, the presented model is compared with experimental data and the simulation results show good agreement with the measurements. Finally, analyses to investigate the influence of the weather conditions, of the axial heat transfer and of the pitch between the turns of the helical pipe for two types of ground are carried out. [Copyright &y& Elsevier]

Abstract: In the present study, a 3D numerical simulation of a whole heat exchanger with middle-overlapped helical baffles is carried out by using commercial codes of GAMBIT 2.3 and FLEUNT 6.3. At first, the computational model and numerical method of the whole heat exchanger with middle-overlapped helical baffles is presented in detail, and parallel computation mode is adopted for the simulation of a whole heat exchanger with six cycles of the middle-overlapped helical baffles of 40° helical angle on a grid system of 13.5-million cells; second, the validation of the computational model is performed by comparing the total pressure drop and average Nusselt number of the whole heat exchanger with experimental data. Reasonably good agreement is obtained, and the reasons causing to the discrepancy are analyzed. The shell-side fluid pressure and temperature fields of the whole area are then presented. Finally the cycle average Nusselt number of different cycle in the heat exchanger are compared and it is found that within the accuracy allowed in engineering computation, periodic model for one cycle can be used to investigate the heat transfer and pressure drop characteristics for different heat exchanger to save computational source. The companion paper will provide details of simulation results using the periodic model for different helical angles and different structures of helical baffles. [Copyright &y& Elsevier]

Abstract: The paper presents the geometry of infinite regular polyhedrons that can be used to construct the heat exchange units. The basic structural component of the infinite solid consists of six hexagons, which meet in the common vertex. The mathematical model of heat and mass transfer inside the labyrinth heat exchanger is shown. The first and the second Kirchhoff’s laws are used to simulate fluid flow distribution at hydraulic network created with connected basic solids. Heat balance equations are adopted to describe forced convection heat transfer inside developed heat exchanger. Good accordance among the experimental data and calculation results over whole simulation range is obtained. The efficiency and thermal characteristic of the labyrinth heat exchanger is presented in the form of graphs. [Copyright &y& Elsevier]