**6. Study of a heat exchanger channel**

416 Heat Exchangers – Basics Design Applications

It must be noted that Focke et al.[15] , who also measured heat transfer coefficients in a corrugated plate heat exchanger having a partition of celluloid sheet between the two plates, reported that the overall heat transfer rate is the 65% of the corresponding value without the partition. **Figure 15** shows that the mean *j-*Colburn factor values calculated using the *overall*  Nusselt number are practically equal to the 65% of the values measured by Vlasogiannis et al. This holds true for all Reynolds numbers except the smallest one (Re=400). In the latter case the Nusselt number is greatly overpredicted by the CFD code. This is not unexpected, since the *two-equation turbulence* model is not capable to predict correctly the heat transfer characteristics for such low Reynolds number.The CFD results reveal that the corrugations enhance the heat transfer coefficient, whereas the pressure losses due to the augmentation of friction factor *f* are increased **(Table 3)**, compared to a smooth-wall plate heat exchanger. Additionally, comparison of the normalized values of Nusselt number and the friction factor, with respect to the corresponding values for the smooth plate (*fsm*, *Nusm*), indicates that as the Reynolds number increases, heat transfer enhancement is slightly reduced, while the friction factor ratio, *f/f* , is increased. This is typical for plate heat exchangers with

Re Nuvlasog 65% Nuvlasog Nu all Nu sm Nu ave/ Nu sm F / fsm 400 13.2 8.6 20.5 - - - 900 38.0 24.7 27.3 9.4 2.9 12.4 1000 41.2 26.8 28.6 10.2 2.8 12.8 1150 44.2 28.7 28.8 11.0 2.7 13.5 1250 46.8 30.4 30.9 11.7 2.7 13.9 1400 49.5 32.2 32.0 12.5 2.6 14.5 Table 3. Experimental values, calculated Nusselt numbers and normalised values of *N*u and *f*

Fig. 14. Comparison of friction factor predictions (CFD) with experimental data.

corrugations [16].

The results for the simplified geometry confirm the validity of the CFD code and strongly encourage the simulation of a module (pass) consisting of two corrugated plates of a compact heat exchanger (**Figure16a**). In order to quantitatively evaluate the results of this simulation, the experimental setup of Vlasogiannis et al.[16] was used as the design model (**Figure 16b**). Due to the increased computational demands, an AMD AthlonXP 1.7GHz workstation with 1GB RAM was used. The geometric characteristics of the new model are presented in **Table 4.** 


Table 4. Geometric characteristics of the model with two corrugated plates.

Preliminary results of the present study, which is still in progress, are shown in **Figure 17**. It is obvious that the herringbone design promotes a symmetric flow pattern (**Figure 16b**). Focusing on the left half of the channel (**Figure 17a**), a close-up of the flow streamlines (**Figure17b**) reveals a "*peacock-tail*" pattern as the liquid flows inside the furrows and over the corrugations. The same flow pattern, which is characteristic for this type of geometry, has also been observed by Paras et al.[14] in similar cross-corrugated geometries (**Figure17c**), where "dry areas" of ellipsoidal shape are formed around the points where the

The Characteristics of Brazed Plate Heat Exchangers with Different Chevron Angles 419

corrugations come into contact. The effect of fluid properties (e.g. surface tension, viscosity) on the shape and the extent of these areas, which are considered undesirable, will be examined in the course of this study.

Fig. 16. (a) Module of a corrugated plate exchanger; (b) The CFD model and (c) Detail of the grid distribution over the corrugated wall.

corrugations come into contact. The effect of fluid properties (e.g. surface tension, viscosity) on the shape and the extent of these areas, which are considered undesirable, will be

Fig. 16. (a) Module of a corrugated plate exchanger; (b) The CFD model and (c) Detail of the

grid distribution over the corrugated wall.

examined in the course of this study.

The Characteristics of Brazed Plate Heat Exchangers with Different Chevron Angles 421

it very difficult to generate an adequate 'database' covering all possible configurations. Thus, CFD simulation is promising in this respect, as it allows computation for various geometries, and study of the effect of various design configurations on heat transfer and

In an effort to investigate the complex flow and heat transfer inside this equipment, this work starts by simulating and studying a simplified channel and, after gaining adequate experience, it continues by the CFD simulation of a module of a compact heat exchanger consisting of two corrugated plates. The data acquired from former simulation is consistent with the single corrugated plate results and verifies the importance of corrugations on both flow distribution and heat transfer rate. To compensate for the limited experimental data concerning the flow and heat transfer characteristics, the results are validated by comparing the overall Nusselt numbers calculated for this simple channel to those of a commercial heat exchanger and are found to be in reasonably good agreement. In addition, the results of the simulation of a complete heat exchanger agree with the visual observations in similar

Since the simulation is computationally intensive, it is necessary to employ a cluster of parallel workstations, in order to use finer grid and more appropriate CFD flow models. The results of this study, apart from enhancing our physical understanding of the flow inside compact heat exchangers, can also contribute to the formulation of design equations that could be appended to commercial process simulators. Additional experimental work is needed to validate and support CFD results, and towards this direction there is work in progress on visualization and measurements of pressure drop, local velocity profiles and

flow characteristics.

geometries.

**8. Appendix Nomenclature** 

D diameter [m] f friction factor G mass flux [kg/m2s]

i enthalpy [J/kg]

j superficial velocity [m/s]

m mass flow rate [kg/s]

heat transfer coefficients in this type of equipment.

Cp constant pressure specific heat [J/kg K]

Ge non-dimensional geometric parameter g gravitational acceleration [m/s2] h heat transfer coefficient [W/m2K]

L c distance between the end plates [m] L h distance between the ports [m] L v vertical length of the fluid path [m] L w horizontal length of the plates [m] LMTD log mean temperature difference [°C]

N cp number of channels for the refrigerant

A heat transfer area of plate [m2] b mean channel spacing [m]

Fig. 17. (a) Streamlines in the left half of the channel; (b) Close up of the flow pattern; (c) Photo of the flow in the cross-corrugated geometry [14].
