Modeling and adjoint optimization of heat exchanger geometries

With the continuous worldwide energy use increase, energy efficiency is gaining high importance. Consequently, many methods have been investigated for potential energy savings. One of these methods is the use of heat recovery systems. These systems basically re-use waste heat and reduce energy consumption. Also, they are increasingly used to reduce heating and cooling demands of buildings. Their main feature is to provide fresh air to the place which is heated by the exhaust air with the help of a heat exchanger (HEX) working between two different temperature sources. The most commonly used types of heat exchangers in ventilation systems are cross-flow and counter-flow heat exchangers. Cross-flow heat exchangers have a thermal efficiency in the range of 50-75% while counter-flow heat exchangers have 75-95%. Many studies have been carried out to increase the efficiency of this type of heat exchangers. In this study, different designs of cross-flow and counter-flow exchangers are compared using ANSYS Fluent software. The aim is to determine how the plate surface geometry affects heat transfer and pressure drop. It is aimed to find the optimum design with maximum efficiency, high heat transfer and low pressure drop for heat exchangers. As a result, it has been observed that thermal efficiency increased from 18% to 60% when changing from cross flow to counter flow in flat plate design, while it increased from 25% to 77% in enhanced plate designs. For enhanced designs, counter flow heat exchanger is 52% more efficient than cross flow heat exchanger. Also, improvements to increase the surface area and turbulence in both flow types have increased heat transfer and thermal efficiency.

An updated review on application of nanofluids in heat exchangers for saving energy

Pressure drop considerations with heat transfer enhancement in heat exchanger network retrofit

Plasma hydrophilic surface treatment for dehumidifying heat exchangers

Numerical analysis of the hydrothermal performance in gasket plate heat exchangers

This paper is intended to overview on analytical and numerical methods in shape optimization. We compute and analyse the shape Hessian in order to distinguish well‐posed and ill‐posed shape optimization problems. We introduce different discretization techniques of the shape and present existence and convergence results of approximate solutions in case of well posedness. Finally, we survey on the efficient numerical solution of the state equation, including finite and boundary element methods as well as fictitious domain methods. Copyright © 2008 John Wiley & Sons, Ltd.

Experimental investigation of effect of different tube row-numbers, fin pitches and operating conditions on thermal and hydraulic performances of louvered and wavy finned heat exchangers

Superfluid at 2 K or below is readily obtained from liquid helium at 4.2 K by reducing its vapour pressure. For better cooling performance, however, the cold energy of vaporized helium at 2 K chamber can be effectively utilized in a recuperator which is specially designed in this paper for accomplishing so-called the distributed Joule-Thomson (JT) expansion effect. This paper describes the design methodology of distributed JT effect heat exchanger for 2 K JT cooling device. The newly developed heat exchanger allows continuous significant pressure drop at high-pressure part of the recuperative heat exchanger by using a capillary tube. Being different from conventional recuperative heat exchangers, the efficient JT effect HX must consider the pressure drop effect as well as the heat transfer characteristic. The heat exchanger for the distributed JT effect actively utilizes continuous pressure loss at the hot stream of the heat exchanger by using an OD of 0.64 mm and an ID of 0.4 mm capillary tube. The analysis is performed by dividing the heat exchanger into the multiple sub-units of the heat exchange part and JT valve. For more accurate estimation of the pressure drop of spirally wound capillary tube, preliminary experiments are carried out to investigate the friction factor at high Reynolds number. By using the developed pressure drop correlation and the heat transfer correlation, the specification of the heat exchanger with distributed JT effect for 2 K JT refrigerator is determined.

Integrating thermal energy storage (TES) systems with high-temperature heat pumps (HTHPs) combines two highly efficient technologies to develop a robust energy storage and flexible heat supply system. This integration package is predominantly valuable for industrial processes, renewable energy storage and supporting the grid where a consistent and intermittent demand for high-temperature heat (above 300°C) exists. Stirling cycle-based heat pumps offer market readiness for high temperature applications with present capabilities attain above 250°C with ongoing developmental campaigns to further increase the temperature thresholds. To achieve greater productivity in thermal storage processes, higher magnitudes of heat transfer temperature are preferred, wherein the heat transfer fluids such as molten salt (MS) are favoured. Molten salts TES is a mature and effective technology for storing large amounts of thermal energy for the emerging high temperature applications in concentrating solar power plants (CSP). In order to facilitate a sustainable operation of a seamlessly integrated TES and HTHP system, development of robust heat exchangers is key to withstand the high temperatures and maintain an effective heat transfer performance of the system. This study aims at optimizing the design of a Stirling cycle based HTHP integrated compact shell and tube heat exchanger that operates between pressurised helium gas from the HTHP side and a ternary molten salt from the TES side. A two-dimensional computational model is developed in ANSYS Fluent with a triangular tube pattern that enables the flow of pressurised helium gas while MS flows around the shell side. Numerical results are performed based on a geometric design optimization to identify the optimal values that influence the fluid and heat transfer characteristics of MS in a shell and tube heat exchangers with a heating capacity of up to 100 kW. A shell side heat transfer coefficient (hsh) and pressure drop (ΔP) are the key parameters of interest to analyse the heat exchanger characteristics with a primary focus on the shell side flow.

Heat transfer and pressure drop performance of twisted oval tube heat exchanger

Cross flow micro heat exchangers were designed to maximize heat transfer from a liquid (water) to a gas (air) for a given frontal area while holding the pressure drop across the heat exchanger of each fluid to values characteristic of conventional scale heat exchangers (car radiators). The cross flow micro heat exchanger design is unique since it produces a very high rate of heat transfer/volume between dissimilar fluids with low fluid pressure drops. Predicted performance for two types of cross flow micro heat exchangers is compared to each other and to current innovative car radiators. Although the cross flow micro heat exchanger has a lower rate of heat transfer/frontal area, it can transfer more heat/volume or mass than existing heat exchangers within the context of the design constraints specified. This can be important in a wide range of applications (automotive, home heating, and aerospace). One type of cross flow micro heat exchanger was fabricated by aligning, then bonding together, two plastic parts that had been molded using the LIGA process. The other type of heat exchanger was fabricated by first electroless plating a layer of nickel over a plastic part molded using the LIGA process, then dissolving the plastic. Performance of the fabricated heat exchangers was measured and compared to the predicted performance. At the design constraints, the mathematical model predicted the fluid pressure drops and heat transfer performance within 10% of the measured performance for the fabricated cross flow micro heat exchangers.

In this study, the additive manufacturing technique has been utilized to fabricate air-water heat exchangers for the application of thermoelectric power plants. Additive manufacturing is a powerful fabrication method that has enabled fabrication of complex geometries that are either challenging or impossible to fabricate based on conventional techniques. Three manifold-microchannel heat exchangers with different interior designs were fabricated by additive manufacturing and from stainless steel. The heat exchangers were tested at different air flow rates and different inlet water temperatures. One heat exchanger was designed and fabricated based on an original design of the manifold-microchannel heat exchanger. Two other heat exchangers were designed with some modifications compared to the original design. In one modified heat exchanger, cylindrical pin arrays were considered on air manifold walls in order to enhance air disturbance, and thus, increase heat transfer between water and air. The second modified heat exchanger contained same pins and also had microchannels in the perpendicular orientation compared to the original design in the outlet manifolds. This design modification was done in order to reduce air-side pressure drop in the heat exchanger. The heat transfer characteristics along with air-side pressure drop were measured and compared with the original design of the manifold-microchannel heat exchanger. Results indicated that the heat flow rate, convection heat transfer coefficient, and pressure drop did not significantly change in modified heat exchangers. For air Reynolds number between around 800 and 4,000, the heat flow rates obtained in the original heat exchanger (type A) and for 50° C water inlet temperature were between 63.9 and 228 W for the lowest and the highest air flow rates, respectively. For the same inlet water temperature, these heat flow rates were between 64.2 and 211 W for the lowest and the highest air flow rates and in one of the modified heat exchangers (type B), respectively. Similarly, while the highest air-side pressure drop in the original heat exchanger was 3458 Pa, this property was measured at 3525 (type B) and 3884 (type C) for the two modified heat exchangers.

Convection heat transfer in a shell-and-tube heat exchanger using sheet fins for effective utilization of energy

A compact and efficient heat exchanger for exhaust gas recovery energy was needed to raise the total efficiency of a thermo-photovoltaic system TPV (Thermo-Photo-Voltaic) for automotive applications (see [1]). In order to respect the strict condition of a high heat transfer surface to volume ratio, a heat exchanger configuration with a plate compact multi-pass counter flow and locally cross-flow recuperator has been chosen. The goal of this work is the understanding of the behaviour of the heat exchanger with numerical and experimental analysis for different geometrical and operating conditions. A high number of dimensions and manufacturing constraints was evaluated before reaching a definite design of a compact and efficient heat exchanger to be tested in the lab for initial experiments. The experimental work was needed in order to validate the numerical model. As the material needed for the real application could not be easily manufactured and instrumented in a workshop, a simplified real model, made of brass, was built, in order to compare numerical results and experimental findings. It was supposed that results obtained in this way would be sufficient to be considered valid when extrapolated in the real heat exchanger high temperature operating conditions and manufacturing material. The experimental results have been successfully compared with numerical ones obtained with the Fluent CFD code (release 6.2.16) Curves of performance (ε-NTU diagram plotted as a function of the ratio between the minimum and the maximum thermal capacities of the flows and pressure drop -mass flowrate diagram as a function of the average temperature) have been obtained and were useful to choose the adequate configuration for different applications, depending on the requested heat transfer and maximum allowable pressure drop. The output of the investigation was: heat transfer, outlet temperatures for both air flows, heat exchanger efficiency, differential pressure drop for both hot and cold sides. After this validation final numerical simulations have been carried out in order to understand the dependence of the heat exchanger efficiency on other geometrical parameters and operating conditions such as plates dimensions, numbers and height of vanes, operating pressure and so on.

Condensation heat transfer and pressure drop of HFC-134a in a helically coiled concentric tube-in-tube heat exchanger