Compact Cross-flow Heat Exchangers Research Articles

Heat transfer effectiveness characteristics maps for additively manufactured TPMS compact heat exchangers

Increased power density in modern miniaturized electronics caused difficulty in keeping electronic performance effective. This challenge leads to the search for high-performance compact heat exchanger as one of the thermal management solutions. Conventionally manufactured heat exchangers had limitations that thwart the development of geometrically complex heat exchangers which are capable of exploiting topological aspects to enhance thermal performance. Subsequently, additive manufacturing (AM) is proposed as a powerful fabrication technique for compact heat exchanger based on the mathematically known triply periodic minimal surfaces (TPMS). In this work, we present 3D compact crossflow heat exchanger computational fluid dynamics (CFD) modelling of geometrically complex structures based on TPMS using STARCCM+ CFD platform. Moreover, CFD modelling is used to obtain new characteristics maps that relate heat transfer effectiveness (Ɛ) and number of transfer units for the proposed heat exchanger. The convection heat transfer coefficient, pressure drop, and inlet and outlet fluid temperature are all examined.

Experimental investigation and modelling of steam-heated supercritical CO2 compact cross-flow heat exchangers

As an answer to the reactor accidents in Fukushima, an additional safety concept is currently under development. The aim is a retrofitting of existing and installation in new nuclear power plants. The new concept is a self-propelling supercritical CO2-operated decay heat removal system based on the Brayton cycle. To be able to prove the functionality of such a system or to investigate next-generation power cycles, existing thermal-hydraulic system codes have to be extended and experimentally validated. In this publication, a novel modelling approach for thermal-hydraulic system codes is presented and validated. For model validation, we take results from our 82 experiments including heat transfer and pressure drop characteristics involving two different compact cross-flow heat exchangers with straight channel dimensions of 2 × 1mm and 3 × 1mm (width × height). On the heat source side, condensing steam flows downwards and on the heat sink side, sCO2 flows horizontally at pressures of 9.50MPa and 7.75MPa. At 9.50MPa, the experimental results of the 3 × 1mm CHX show a better thermal-hydraulic performance than the 2 × 1mm CHX due to an improvement of the heat transfer rate of up to 6% and an approximately 20% lower pressure drop. The experiments at 7.75MPa show a slight increase in the heat transfer rate compared to 9.50MPa. By modelling just one representative plate and additional agglomeration of subvolumes, the approach enables fast and accurate calculations. Compared to the experimental results, most of the simulated cases show a deviation of less than 8% in terms of the heat transfer rate. The simulated pressure drop is in all cases within 92% of the experimental error band. This approach also enables to simulate large-scale cross-flow CHXs with limited numerical effort (low number of subvolumes). However, the impact of mass flow maldistribution, the choice of modelling options, and the effect of thermodynamic property variations need to be analysed in more detailed further investigations.

Steady RANS methodology for calculating pressure drop in an in-line molten salt compact crossflow heat exchanger

The development of molten salt cooled reactors (MSR) and fluoride-salt cooled high temperature reactors (FHR) requires the use of advanced design tools for the primary heat exchanger (PHX) design. Due to geometric and flow characteristics, compact (pitch to diameter ratios equal to or less than 1.25) heat exchangers with a crossflow flow arrangement can become desirable for these reactors. Unfortunately, the available experimental data is limited for compact tube bundles or banks in crossflow. Computational Fluid Dynamics (CFD) can be used to alleviate the lack of experimental data in these tube banks. Previous computational efforts have been primarily focused on large pitch to diameter ratios (larger than 1.4) using unsteady Reynolds averaged Navier-Stokes (RANS) and Large Eddy Simulation (LES) frameworks. These approaches are useful, but have large computational requirements that make comprehensive design studies impractical. A CFD study was conducted with steady RANS in an effort to provide a starting point for future design work. The study was performed for an in-line tube bank geometry with FLiBe (LiF-BeF2), a frequently selected molten salt, as the working fluid. Based on the estimated pressure drops, the pressure and velocity distributions in the domain, an appropriate meshing strategy was determined and presented. Periodic boundaries in the spanwise direction transverse flow were determined to be an appropriate boundary condition for reduced computational domains. The domain size was investigated and a minimum of 2-flow channels (FC) for a domain is recommended to ensure the behavior is accounted for. Lastly, the standard low Re κ-ε (Lien) turbulence model was determined to be the most appropriate for steady RANS of this case at the time of writing.

A hybrid method for the cross flow compact heat exchangers design

Despite their limitations, the cross flow compact heat exchangers are generally modeled by the ε-NTU and LMTD methods and this mainly leads to the absence of effective consideration on the heat transfer geometry at the micro scale. At the same time, numerical analysis applied to compact cross flow heat exchangers, having different and complex finned surfaces respectively at the hot and cold sides, involves high computational costs.A powerful alternative design procedure is here proposed that takes advantage of both numerical and analytical approaches. Hot and cold sides are numerically modeled and predictor functions for heat transfer and fluid dynamic performance are obtained with regression technique, for both sides.The whole cross flow heat exchanger is divided into a set of control volumes, including the fins geometry 3D accurate description of both sides and their separation wall. An analytic iterative method is then used to find a wall temperature distribution throughout and to determine the mass flow rate distributions on both sides starting from the results of the numerical analysis at the micro scale.The multi-scale approach leads to a better accuracy level with respect to the full-scale one and allows to profitably investigate different fins influence on flow distributions, local heat transfer and pressure losses through both sides of the heat exchanger.

Theoretical investigation on cross-flow heat exchangers with axial dispersion in one fluid

The axial dispersion model for cross-flow heat exchangers is investigated to predict steady state thermal performance more accurately. This model takes flow maldistribution and backmixing into account and is simple and effective in describing their effect on the temperature effectiveness of a heat exchanger. An exact solution together with its asymptotic form for high dispersive Péclet numbers is obtained. The analysis shows that the temperature effectiveness deterioration caused by axial dispersion is significant for the dispersive Péclet number P e < 20, particularly when the thermal capacity rate ratio is close to one. The P 1, P 2-charts are presented, which are useful for the design of compact cross-flow heat exchangers.

Thermal performances of heat exchangers applicable to waste-heat recovery systems

Results are presented and discussed for the quantitative evaluation of the heat exchangers applied to waste-heat recovery systems for two types of conventional heat exchangers: compact cross-flow heat exchangers, and heat-pipe heat exchangers. The effects of the different alignments on the heat transfer rate of heat pipe heat exchangers are also studied.

One approach to irreversibility minimization in compact crossflow heat exchanger design

The minimum of enthalpy exchange irreversibility (EEI) as a selective criterion in heat exchanger design is considered. This concept is applied in the core sizing procedure of a compact crossflow heat exchanger for gas-to-gas application. This approach objectivizes, in the final analysis, the pressure drop choice in such a way that from the total set of possible heat exchanger core dimensions the thermodynamically optimal one is selected.

Construction of and Measurements with an Extremely Compact Cross-Flow Heat Exchanger

Abstract An extremely compact cross-flow heat exchanger is described. The exchanger is constructed by furnace-brazing together a stack of hundreds of stainless-steel sheets. The resulting structure is leak-tight and very strong, but fluid channels as small as 51 μm (0.002 in) are not plugged by excess brazing material. Measurements of heat transfer between water and liquid propylene flowing through the heat exchanger are in excellent agreement with calculations based on exchanger geometry and fluid properties.

Fabrication and leak-tight furnace brazing of intricate objects

An extremely compact crossflow heat exchanger has been constructed by hydrogen furnace brazing together a stack of hundreds of chemically milled stainless-steel sheets. The resulting structure is leak tight and very strong, but fluid channels as small as 51 μm are not plugged by excess brazing material. The construction technique is easily adapted to mass production and should be useful for structures of an intricate but repetitive three-dimensional nature.