**2.1 Test station**

120 Heat Exchangers – Basics Design Applications

and next calculated the heat exchange effectiveness and the thermal performance deterioration factor with finite difference scheme. Experiments were performed to validate the flow distribution and heat transfer model. The results indicate that when the channel pitch is below 2.0 mm, the flow distribution is quite homogeneous and the thermal deterioration due to flow maldistribution can be neglected. However, when the channel pitch is larger than 2 mm, the maldistribution is quite large and a 10–20% thermal

This literature review of the selected positions shows, as already mentioned, that the problem of the non-uniform fluid inflow to the heat exchangers has been the subject of many computational and experimental investigations, but the results obtained are unambiguous in terms of thermal performance. Many investigations are limited to the hydraulic analysis only and they deal with liquid-liquid type heat exchangers. Most researchers are consistent in finding that the non-uniformity of the flow significantly strikes the hydraulic efficiency of heat exchangers. Thermal analyses refer first of all to the heat exchanger effectiveness, but they are not very numerous. It is lack of complete investigations of the finned cross-flow heat exchangers of the gas-liquid type with unequal

The degradation effects of flow maldistribution on the performance of a heat exchanger are well-known. Not only does the thermal performance decrease but the fluid pressure drop across the exchanger core also increases simultaneously. Analyzing the results of (Piątek, 2003) the obvious question has appeared: how reliable are these results? The HEWES code validation procedure has to be carried out in order to answer this question. It became possible after modernization of the experimental rig and installation of the hot water supply module. In (Bury et al., 2007b) there have been presented the only initial results obtained by use of the modified testing station, and the results of initial and detailed validation and sensitivity analysis have been presented in (Bury et al., 2008a)) and (Bury et al., 2008b). Significant differences have been recorded between experimental and numerical data after the initial validation of the model. Minor changes have been put into the code and the validation procedure was then repeated with usage of the infra-red thermography measurements results also. The last stage of the research was the sensitivity analysis. This analysis has shown that the heat transfer coefficient from ribbed surfaces to a gas may be a reason for recorded discrepancies between numerical and experimental results. An additional testing station, in the lab-scale, has been designed and constructed in order to check the numerical procedure responsible for determination of the heat transfer coefficient from the ribs to the gas. The papers (Bury et al., 2009a; Bury and Składzień,2010) and recently also (Składzień and Bury, 2011) present results of this

Applying the validated version of the HEWES code and modified testing station the analysis of the above mentioned car cooler has been repeated and the results allowed to sustain the conclusions withdrawn by Piątek – the air inflow maldistribution may significantly affect

The following questions have emerged after analysis the experience gained so far:

deterioration factor could be found.

inflow of the agents, especially of unequal inflow of the gas.

**1.3 Aim and scope of presented studies** 

the heat exchanger performance (Bury et al., 2009b).

analysis.

The test station consists of two main modules: the air supply module (see Fig. 1) and the hot water supply module (Fig. 2). The air supply module originally was a special testing station constructed during realization of the project (Piątek, 2003) for determination of a form and scope of the air inflow non-uniformity.

Fig. 1. Test station - the air supply module (1 – support plate, 2 – heat exchanger, 3 – thermoanemometric sensor, 4 – measuring probe, 5 – diffuser, 6 – channel, 7 – control computer, 8 – fan).

The air is supplied by the radial fan of the maximum capacity of 6900 m3/h. The fan capacity can be controlled by the throttling valve installed before the fan. Then the air flows through the 1.7 m long channel (rectangular cross-section 190x240 mm). The channel ends with the filter section. Usually this section is empty and only during special tests filters having the form of wire nets or perforated metal sheets are used. Actually, filter is not a good word describing the purpose of these elements – they are installed in order to make the air flow more uniform. The diffuser dimensions have been fit to the first examined heat exchanger: they are 280x490 mm.

The main element of the measuring system is the V1T-type thermoanemometric sensor installed onto the measuring probe which shifting is controlled by a computer. It allows

Impact of a Medium Flow Maldistribution on a Cross-Flow Heat Exchanger Performance 123

measuring nodes

Fig. 3. The idea of the spiral-type (left) and regular-type measuring meshes and trajectories

The time constant of the measurement and the number of measurements realized in each node should be entered in the file. The data are acquired with the maximum frequency allowed by the hardware (CPU clock). So, assuming a 100 Hz frequency and 0.5 s time constant there would be 50 samples obtained for the given measuring node. The results are analysed online and the output file contains the average values with their standard deviations for each measuring node, considering both velocity and temperature of the

A higher resolution of the results (velocity and temperature distributions) can be achieved by making the measuring meshes more dense. Definition of the measuring mesh needs some optimization between resolution of results and time of measurement, and the aim of measurement as well as the heat exchanger structure should be also taken into account.

A regular measuring mesh of 196 nodes has been used for measurements realized in this work. The measuring program has been started after the steady state conditions were

Three parameters are assumed as independent and may be set by a researcher: the air and

The cooler heat capacity has been determined as the heat flow rate transferred in the exchanger computed from the air and the water side. Obvious relationships describing the

> *QV c t t a aa*

*QV c t t w ww* 

assumed according to thermodynamic tables for the outlet temperature.

The air density has been calculated using the ideal gas law for the absolute pressure and the air average temperature at the inlet to the exchanger. The density of water has been

The water enthalpy drop has been used for calculations of the heat flow rates because of

*pa a out a in* , , (1)

*pw w in w out* , , (2)

of the measuring probe movement.

water flow rates and the inlet water temperature.

more accurate measurement of the water flow.

medium enthalpy decrease (increase) have been used:

air.

achieved.

determining velocity and temperature fields of the air at the exchanger's inlet and outlet. The measuring probe moves in a clit cut out in the upper wall of the diffuser. The clit is seal up with a soft insulating foam. Unfortunately, such a solution is the reason of some air leakage. As the thermoanmenometric sensor is a very fragile instrument its contact with walls and other structures should be prevented. There are 20 mm wide margins left on the all sides and the probe movement plane is placed 25 mm in front of the heat exchanger's inlet cross-section. Signals from the sensor are gathered by the FMC 921 control card and send to the computer where they are analysed.

The original testing station has been modified and the hot water supply module was installed. Water is heated up to the desired temperature (up to 95°C) by the electric heater. The water circulation is forced by the pump and its flow rate can be regulated by the control valve. The flow rate is measured by the rotameter and the K-type thermocouples (NiCr-NiAl) measure its temperature at the inlet and outlet of the heat exchanger.

Fig. 2. Test station - the hot water supply module (1 – electric heater, 2 – cut-out valve, 3 – manometer, 4 – control valve, 5 – heat exchanger, 6 – temperature measuring system, 7 – flow meter, 8 – pump).

The measuring system allows for acquisition of the following parameters at the moment: total air volumetric flow, the water mass flow rate, inlet and outlet water temperature, distribution of the air velocity and temperature at the inlet and outlet of the heat exchanger.

#### **2.2 Procedures of measurements and experimental data analysis**

The air temperature and velocity distributions measurement need the measuring task to be defined in the form of an input file for the program controlling the measuring probe's work. The trajectory of the probe's shifting is determined by location of measuring nodes. There are two ways for realizing the measurements: applying the spiral-type measuring mesh or the regular-type mesh. These two types of measuring meshes are shown in Fig. 3. The first one is usual while determining the form and scope of the air inlet non-uniformity. Data obtained by use of the regular mesh are more convenient for the complete thermodynamic analysis. Such mesh divides the whole measuring cross-section into identical rectangles and the measuring nodes are located in the middle of each rectangle.

determining velocity and temperature fields of the air at the exchanger's inlet and outlet. The measuring probe moves in a clit cut out in the upper wall of the diffuser. The clit is seal up with a soft insulating foam. Unfortunately, such a solution is the reason of some air leakage. As the thermoanmenometric sensor is a very fragile instrument its contact with walls and other structures should be prevented. There are 20 mm wide margins left on the all sides and the probe movement plane is placed 25 mm in front of the heat exchanger's inlet cross-section. Signals from the sensor are gathered by the FMC 921 control card and

The original testing station has been modified and the hot water supply module was installed. Water is heated up to the desired temperature (up to 95°C) by the electric heater. The water circulation is forced by the pump and its flow rate can be regulated by the control valve. The flow rate is measured by the rotameter and the K-type thermocouples (NiCr-

**1 23 4 5**

Fig. 2. Test station - the hot water supply module (1 – electric heater, 2 – cut-out valve, 3 – manometer, 4 – control valve, 5 – heat exchanger, 6 – temperature measuring system, 7 –

The measuring system allows for acquisition of the following parameters at the moment: total air volumetric flow, the water mass flow rate, inlet and outlet water temperature, distribution of the air velocity and temperature at the inlet and outlet of the heat exchanger.

The air temperature and velocity distributions measurement need the measuring task to be defined in the form of an input file for the program controlling the measuring probe's work. The trajectory of the probe's shifting is determined by location of measuring nodes. There are two ways for realizing the measurements: applying the spiral-type measuring mesh or the regular-type mesh. These two types of measuring meshes are shown in Fig. 3. The first one is usual while determining the form and scope of the air inlet non-uniformity. Data obtained by use of the regular mesh are more convenient for the complete thermodynamic analysis. Such mesh divides the whole measuring cross-section into identical rectangles and

**2.2 Procedures of measurements and experimental data analysis** 

the measuring nodes are located in the middle of each rectangle.

**<sup>7</sup> <sup>6</sup> <sup>8</sup>**

**oC**

NiAl) measure its temperature at the inlet and outlet of the heat exchanger.

send to the computer where they are analysed.

flow meter, 8 – pump).


measuring nodes

Fig. 3. The idea of the spiral-type (left) and regular-type measuring meshes and trajectories of the measuring probe movement.

The time constant of the measurement and the number of measurements realized in each node should be entered in the file. The data are acquired with the maximum frequency allowed by the hardware (CPU clock). So, assuming a 100 Hz frequency and 0.5 s time constant there would be 50 samples obtained for the given measuring node. The results are analysed online and the output file contains the average values with their standard deviations for each measuring node, considering both velocity and temperature of the air.

A higher resolution of the results (velocity and temperature distributions) can be achieved by making the measuring meshes more dense. Definition of the measuring mesh needs some optimization between resolution of results and time of measurement, and the aim of measurement as well as the heat exchanger structure should be also taken into account.

A regular measuring mesh of 196 nodes has been used for measurements realized in this work. The measuring program has been started after the steady state conditions were achieved.

Three parameters are assumed as independent and may be set by a researcher: the air and water flow rates and the inlet water temperature.

The cooler heat capacity has been determined as the heat flow rate transferred in the exchanger computed from the air and the water side. Obvious relationships describing the medium enthalpy decrease (increase) have been used:

$$
\dot{Q}\_a = \dot{V}\_a \cdot \rho\_a \cdot \mathbf{c}\_{pa} \cdot \left(t\_{a,out} - t\_{a,in}\right) \tag{1}
$$

$$
\dot{Q}\_w = \dot{V}\_w \cdot \rho\_w \cdot \mathcal{c}\_{pw} \cdot \left(t\_{w,in} - t\_{w,out}\right) \tag{2}
$$

The air density has been calculated using the ideal gas law for the absolute pressure and the air average temperature at the inlet to the exchanger. The density of water has been assumed according to thermodynamic tables for the outlet temperature.

The water enthalpy drop has been used for calculations of the heat flow rates because of more accurate measurement of the water flow.

Impact of a Medium Flow Maldistribution on a Cross-Flow Heat Exchanger Performance 125

An attempt for systemizing this non-uniformity has been undertaken in (Malinowski, 2008). The numerical analysis has proved that the reason of the observed air inflow maldistribution is the radial fan. Unfortunately, attempts to describe the measured inequality by using mathematical functions have failed. For this reason, data on the nonuniformity are included in the calculations in tabulated form using rows. This extends the calculation time slightly, but on the other hand allows for accurate recognition of this

Fig. 5. Distribution of the air velocity at the inlet (left) and outlet (right) cross-sectional flow

Fig. 6. Distribution of the air temperature at the inlet (left) and outlet (right) cross-sectional

Fig. 7. Distribution of the air velocity at the inlet cross-sectional flow area (210mm x 400mm) of HE-2/1 measurement (left – without throttling) and of HE-3/4 measurement (right –

Presented in Figs. 5-7 distributions of velocity and temperature of the air were drawn as

area (210mm x 400mm) of HE-1/1 measurement, m/s.

flow area (210mm x 400mm) of HE-1/1 measurement, °C.

maximum throttling), m/s.

viewed from the outlet of the heat exchanger.

phenomenon.

## **2.3 Analysed heat exchanger types**

The investigations accomplished in this work deal with the ribbed cross-flow heat exchangers of the gas-liquid type. There were three water coolers investigated during realization of this work (see Fig. 4):

HE-1 – typical car cooler (Skoda Favorit 135L) with the core having the form of 2 rows pipe bundle (15 cylindrical pipes ribbed with the plate fins in each row, 380 fins on each pipe); aluminium,

HE-2 – the cross-flow heat exchanger made by GEA Heat Exchangers Company with the core made of 10 rows of elliptical pipes ribbed with the plate fins (175 on each pipe); steel,

HE-3 - the cross-flow heat exchanger made by GEA Heat Exchangers Company with the core having the form of 2 rows pipe bundle (81 fins on each pipe in the first row and 140 fins on each pipe in the second row); steel.

Fig. 4. General sketch of the heat exchangers under consideration and the recurrent elements of three versions of the heat exchangers
