El modelo de Daniel Goleman

Subsystem

Side effects Power consum

Engine Speed

Flow to Componen

Failure Modes Noise

E i Legend V = Component varia

A = Aging U = Customer usag

E = Environmenta

Part 2 - Section 3

External Flow Subsystem

Figure 31, Fin and tube heat exchanger performance

DEGAS FLOW REQUIREMENTS

De-aeration (or “degas”) reservoirs require a minimum flow rate for adequate performance. There is also a maximum flow limitation, above which the reservoir will cease to de-aerate and begin to re-de-aerate the coolant. This maximum flow rate can be detected by installing a reservoir on a test rig and steadily increasing the flow rate. The critical flow rate at which the coolant is being re-aerated occurs when the coolant level rises (aeration of the coolant increases the volume), and air bubbles can be observed exiting the reservoir, (requires the installation of a clear hose at the reservoir exit).

NOTE: De-aeration requirements are discussed in a subse-quent chapter.

SYSTEM DESIGN FOR ADEQUATE FLOW

A simple schematic of the type shown in figure 32, can be used to indicate which components are in the system, and how the components are connected. Often more than one schematic will be considered in the initial stages of the system design.

Given the schematic, and the hydraulic resistance of the components, you can determine the required output of the pump. The term “hydraulic resistance” is used here to indicate the pressure loss of the fluid as it flows through the component. More generally, it is the energy required to flow fluid through the component.

The quantitative unit of measurement of hydraulic resistance is the loss coefficient. Loss coefficient is the

non-dimensional difference in pressure across a component. The hydraulic resistance of the component to the flow will cause a disruption in the flow, which will persist beyond the component and into the pipe beyond the component. The total pressure loss attributable to the component then must be measured across the entire distance that this flow disruption occurs.

Figure 32, Cooling System Schematic Example

To measure loss coefficient then, there must be a straight section upstream of the component to ensure uniform, fully developed flow into the component, and a straight section downstream of the component as well. The downstream pressure measurement must be taken sufficiently far downstream that the flow at the location of the pressure measurement is again fully developed. The pressure loss due to the pipe length between the upstream and downstream pressure measurements is subtracted from the pressure difference. In figure 33, the loss coefficient is the difference between the two parallel lines that represent the pressure versus length lines for the straight pipe with no component, and with the component in-line with the pipe.

Figure 33, Loss Coefficient^[14]

Coolant flowrate (kg/s)

Heat transfer coefficient (kW/m²K) Increasing

Airflow

Engine

Water Pump

Radiator

Degas Heater

Thermostat

Bypass

Non-dimensional Total Pressure

Non-dimensional Length

Friction gradient with Zero loss component

Redeveloped friction gradient Developed friction

gradient

Component

∆∆∆∆H U / 2g2 K =

Measuring loss coefficients is often appropriate where existing production parts are being utilized. Estimates of the loss coefficient (based on measurements of similar components) are appropriate for new components in the early stages of system design. When measuring loss coefficient, it is important to have fully developed flow upstream of each pressure measurement location, to eliminate variability in the measurement, and so that the loss associated with redevelopment of the flow is adequately accounted for. Figure 34 shows a schematic of a typical component flow measurement set up.

Figure 34, Set up for component flow measurement

Calculate the loss associated with the flow through the pipe length after the test component and before the second pressure measurement. Then subtract this from the pressure loss measured.

The pump output requirements can be determined after deciding on the flow arrangement and either measuring or estimating the loss coefficients for each component (as well as for the connecting hoses). The mathematical problem is easily defined as illustrated in the following example.

Consider a simplified circuit with a schematic as in figure 35.

Figure 35, Simplified Circuit Schematic

The pressure drop across the two components in Figure 35 is given by the following equation:

(4)

Where: Q is volumetric flow rate

,

ρ is density, A is area, and K_A and K_B are component loss coefficients.

Plotting this curve and determining the point of intersection with the pump curve will provide the operating point.

However parallel circuits, component interactions, and the need to iterate the design will significantly increase the complexity of the task. This process lends itself to computer simulation that can be easily applied with the use of a one-dimensional axial flow simulation code, such as FLOWMASTER.

Although this can be done mathematically, a computer code is extremely useful for this purpose. The input required and the computation time for a one-dimensional flow simulation are both significantly less than would be required for a 3-D or computational fluid dynamics code.

The pump must be matched to the system as well as provide enough output (head) for adequate flow. This means that the operating point must be near the peak efficiency of the pump. If the flow at the operating point exceeds the flow at best efficiency, then the risk of cavitation is significantly increased. Cavitation occurs when cavities or vapor bubbles form and subsequently collapse as they are carried into regions of higher pressure.

As a general guideline, the operating point with the thermostat open should be at the point of best efficiency, and the operating point with the thermostat closed should correspond to a flow rate of no less than 80% of the flow with the thermostat open.

Figure 36, Capacity Range 30 Diameters

30 Diameters

Pressure measurement

locations

Component to be tested

K

^A

K

^B

Component B Component A

Pump

∆P Q²ρ

--- K2A ( _A+K_B)

=

Hydraulic Efficiency

Thermostat Open Thermos

tat Close d

Capacity Range

Best Efficiency Point

Flow Capacity

Static Pressure Difference (Head)

Greater Than 80% BEP

The requirement for pump characteristic curve (or range of curves) is passed along to the pump design engineer or supplier.

After estimating loss coefficients and pump characteristics, the pump designer must verify that these estimates are feasible. If further investigation shows that the component requirements are not likely to be met, then it is necessary to modify the design of the system. Although the requirements process begins at the vehicle level, the process of setting targets is highly iterative and requirements are imposed on the system by the components just as the system imposes requirements on the components. The original function of the cooling system after all, is to provide an environment in which components can survive.

SYSTEM DESIGN ALTERNATIVES

If the required flow rates are not achieved, the following system design alternatives can be considered:

• An external heater bypass (an additional flow branch) will increase flow through the engine, and decrease flow through the heater.

• If heat exchangers are arranged in parallel rather than series, overall flow rate is increased. For example, if an oil cooler is in parallel with the heater rather than in series, this will increase flow through the engine and decrease flow through the heater.

• Reducing hose bends and increasing hose diameters will increase flow.

• A dual acting thermostat, which closes off the bypass as the radiator flow path is opened will increase flow through the radiator.

COMPONENT DESIGN ALTERNATIVES

If system modification fails to produce adequate coolant flow, then the component design must be modified to:

• reduce the hydraulic resistance of the components,

• or increase the output of the pump,

• or decrease the flow requirements of the components.

In addition to minimum flow requirements, there are also maximum flow requirements, particularly with heat exchangers and degas reservoirs. Sometimes the minimum and maximum flow requirements for heat exchangers cannot both be accomplished without the addition of a diverter valve. The diverter valve causes some of the flow to bypass the heat exchanger at high pump speeds. For degas reservoirs, a restrictor is generally used to reduce the flow rate to the reservoir. Any restrictor in the degas reservoir inlet line should be added to the line as far

away from the reservoir as possible, as illustrated in figure 37. This is desirable for two reasons:

• Flow as it leaves the restrictor has a high local velocity, but further downstream the flow redevelops at a lower velocity (the velocity of the fluid entering the reservoir should be as low as possible).

• The hose and any hose joints downstream of the restrictor will be subjected to pressures that are lower than those locations which are upstream of the restrictor.

Figure 37, Flow restrictor location

VERIFICATION

A validated one-dimensional computer simulation code can be relied upon for much of the product development process. To validate a flow simulation, the output of the flow model should be compared to the results of a bench test.

The difference between the simulation and the bench test must be well below the safety margin applied.

For example, if the critical flow rate through the heater is 20 gpm, and you have designed the system to deliver no more than 15 gpm, (a 25% safety margin), then a 5% to 10% degree of accuracy for the computer model is likely to be acceptable.

When you conduct a flow test, the requirements of the flow measurement devices must be observed. The flow meter may require a length of straight section upstream (and sometimes downstream) in order to accurately measure flow. Furthermore most flow meters provide a measurable hydraulic resistance which will alter the resultant flow. For best results, set up a flow stand which accommodates all of the requirements of each of the flow and pressure measurement devices (there can be flow development, temperature, and vibration limitations). Then simulate the very same circuit (including all measurement devices) in the computer model.

Engine

Radiator

Degas

Restrictors should be added at these locations Restrictors should

NOT be added at these locations

If the output of the flow stand and the flow model are sufficiently close, then modify the computer model so that the hydraulic resistance of any measurement devices is removed. The results are then corrected flow stand data. If the computer model shows consistent agreement to the flow stand, then additional design alternatives can be evaluated with the computer model.

OBJECTIVE

The objective of the heat dissipation subsystem is to protect the coolant by keeping it liquid at all times. This is achieved by ensuring that the bulk coolant is not allowed to reach a temperature where a local heat flux, anywhere within the engine, is sufficient to cause the coolant to boil.

In essence, the coolant protects the engine, and the heat dissipation system protects the coolant.

In this section we will discuss:

In document LA INTELIGENCIA EMOCIONAL (página 65-69)