The determination of the optimal circulation fluids speeds through the heat exchangers.

Nagi, Mihai ; Ilies, Paul ; Negoitescu, Arina 等

1. INTRODUCTION

In order to choose a heat exchanger from various possible solutions it would be preferred that device (Badea & Necula, 2000) which transmits the constrained heat flow Q with a minimum consumption of energy (power P) for the two fluids circulation through the device, and which occupies the lower volume.

The decrease of the heat exchanger volume can be obtained by increasing the circulation speeds of the two fluids (for an existing device) or modifying the heat transfer surfaces by choosing surfaces with turbulence generators (Kays & London, 1984). Both methods lead to the increase of the consumed power for fluids circulation.

The thermal flow transmitted between the two fluids into a heat exchanger depends almost entirely on the two fluids speeds whether their entrance temperatures are constant. The increase of the fluids circulation speeds also increases the required power for fluids circulation.

Under these circumstances the problem is to establish the optimal circulation speeds of the fluids in order to obtain a solution as economic as possible for the selected device (Badea & Necula, 2000; Negoitescu, 2003).

2. THE COMPARISON OF THE HEAT EXCHANGERS

In order to compare various designs of heat exchangers are used the following criterions (Nagi & Negoitescu, 1998; Badea & Necula, 2000):

--The total thermal transfer coefficient.

It is a basic criterion according to which the thermal flow is related to the heat transfer surface A and the medium temperature difference [DELTA][t.sub.m] between the two fluids.

k = [??]/A x [DELTA][t.sub.m] [W/[m.sup.2]K] (1)

--The volume efficiency "[[epsilon].sub.v]"

If in relation (1) the surface of heat transfer is replaced by the volume V [[m.sup.3]] it will be obtained:

[[epsilon].sub.v] = [??]/V x [DELTA][t.sub.m] = k x A/V [W/[m.sup.2]K] (2)

Both criterions of comparison give information about the thermal performances by the total thermal transfer coefficient but give no information about the consumed energy in order to obtain this coefficient value.

--The energetic efficiency "[[epsilon].sub.e]"

Under these circumstances the thermal flow transmitted for a temperature difference between the two fluids equal to one degree is related to the requisite power for fluids circulation through the device:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (3)

Where:

[[??].sub.i] [[m.sup.3]/s]-the fluids volume flows;

[DELTA][p.sub.i] [Pa]-the fluids pressure losses (i=1,2).

The last criterion can be rigorous applied only when the thermal power ([??]/[DELTA][t.sub.m]) of the heat exchangers for the constrained volume flows is the same.

From this point forward the optimal circulation speed of the two fluids represents the pair of values ([W.sub.1], [W.sub.2]) that leads to the maximum values of the energetic or volume efficiency.

As the pressure losses in compact heat exchangers on the oil or water side are relative large the requisite power for fluids circulation is large too.

3. EXPERIMENTAL RESEARCHES

By experimental researches were established he optimal circulation speeds of water and oil through an oil cooler with corrugated ribs on the oil side (21 oil tubes).

In Figure 1 is plotted the energetic efficiency variation versus the oil circulation through the cooler tubes. It can be observed that to a single speed of the oil corresponds a single speed of the water for which the energetic efficiency is maximal. Thus, the optimal values ([w.sub.u], [w.sub.w]) of oil and water are obtained.

The less the oil speed is, the more important is to choose correctly the value of the water speed. For an oil speed between 0.6 ... 0.8 m/s, the economic efficiency can decrease below the half optimal value, if the value of the water speed is too large. The water and oil circulation speeds can not be independently selected. Each oil speed corresponds to a single economic value of the water speed.

The optimal circulation speeds of water and cooling fluid for a car radiator with corrugated ribs on the air side were also established by experimental researches (Nagi & Lelea, 1997). In Figure 2 is plotted the energetic efficiency variation versus the air circulation speed through the radiator.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

From Figure 2 it can be observed that a single oil speed has a corresponding single economic water speed. The less the air speed is, the more important is to choose correctly a value for the water speed.

In Figure 3 is plotted the energetic efficiency variation versus the volume efficiency for a car radiator. The optimal circulation speeds of water and air result from the diagram which is presented in Figure 3.

4. CONCLUSION

For a heat exchanger selection, from various possible solutions, it will be preferred that device which transmits the constrained thermal flow for a price as low as possible, works under safety conditions and occupies the smaller volume.

[FIGURE 3 OMITTED]

Due to the complex phenomenon that appear in the case of compact heat exchangers, only the experimental researches can answer the questions regarding the thermal and fluidodynamic performances of these types of devices. The optimal circulation speeds through the device can be established by experimental researches. The circulation speeds of fluids can not be independent chosen. By breaching this optimal correlation can lead to important decrease of the energetic efficiency value in comparison with the maximum value.

The experimental results that were obtained and presented above can be used by researchers only for devices and similar working conditions.

5. REFERENCES

Badea, A., Necula, H. (2000), Schimbatoare de caldura. (Heat Exchangers), Editura A.G.I.R., Iasi

Kays, M., London, A. (1984), Compact Heat Exchangers, Third Edition, McGraw-Hill.

Nagi, M., Lelea, D. (1997), Experimental investigation on heat exchangers for automobiles. MVM Proceedings, Vol. 23, pp. 45-47, Kragujevac

Nagi, M., Negoitescu, A.S. (1998), Calculul si constructia instalatiilor termice (Design and Development of Thermal Equipments), Editura "Eftimie Murgu". ISBN 973-97754-97, Resita

Negoitescu, A.S. (2003), Researches about the Heat Transfer in the Heat Exchanger Extend Surfaces, 6th International Conference on Accomplishments of Electrical and Mechanical Industries DEMI, ISBN 99938-623-8-X, Banjaluka