DD284282A5 - METHOD AND DEVICE FOR MEASURING EVAPORATION - Google Patents

Abstract

The invention describes a method and a device for measuring the evaporation mass flow density on the surface of condensed phases, which are free or capillary bound. The basis for such a measurement is the enrichment of a tracer gas with the vapor of the evaporating phase, whereby the target mass is connected via the continuity equation of the continuum mechanics with the state change of the carrier medium. Fig. 3 {evaporation mass flow density; Evaporation measurement; transpiration; Evapo-transpiration; Sublimationsmessung; condensed phase; capillary liquid}

Description

For this three pages drawings

Field of application of the invention

The invention relates to those tasks of the measuring technique, which have the measurement of the evaporation of condensed phases in Evaporations-, evapotranspiration, transpiration and sublimation phenomena the subject. In particular, the invention opens up application areas in physics, chemistry, biology, medicine, meteorology, in construction and in the process control of the wood, paper, textile and food industries.

Characteristic of the known solutions

In the solutions known hitherto, either evaporative phase properties or physical variables coupled with the evaporation process are investigated to determine the evaporation.

Thus, the prior art techniques are tied to certain model ideas about the nature of the evaporating phase, such as state of matter, chemical composition or physical and chemical state of binding, and the evolving evaporation mechanism for quantifying transport sizes.

Object of the invention

The aim of the invention is the in-situ measurement of the evaporation mass flow density over condensed free or bound phases.

Explanation of the essence of the invention

The object of the invention is, regardless of the P'.zeugung the evaporating phase and outside of the same by measuring a directly identifying the evaporation mechanism physical size to capture the evaporation process quantitatively.

If evaporation phenomena occur on the surface of condensed phases, this results in a vapor flow into the environment of the test object.

According to the invention, a gaseous carrier phase is used which overflows the surface of the evaporating phase, absorbs this vapor stream and thereby changes its thermodynamic state in a defined manner.

By following the change in the state of the carrier medium while taking into account the continuity equation of the continuum mechanics for the evaporation mass flow densities, a regulation for the determination of the evaporation can be derived:

a) - after integrating procedures

In the defined constant volume V _{m of} the carrier gas is on the Vei discontinuity A _v the evaporation mass flow density jy directed (Figure 1)

 The continuity equation in the form applies:

Ao Av

-JjTpM

If the state variables T and φ of the carrier medium are measured at the times t 'and t' + At, this results from consideration of

or by using the state equation of ideal gases and with t '= 0, At = t _m

By means of a corresponding measuring arrangement, it is therefore possible to determine the evaporation mass flow density from the temperature T and the relative vapor saturation φ of the carrier medium at times t = 0 and t = t _m using the specified continuity equation of continuum mechanics. B) - by the differential method

In the measuring time t _m, a defined volume of the carrier medium V _n , = v <Ir) Ak (r) tm flows over the evaporation surface A _v and thereby absorbs the vapor flow j _v (Figure 2).

The continuity equation (1) takes the form for this case under the condition pd = pd (r)

ae Av A *

β'j *

dT - 0 W

It follows that with j = j (t) and j ,,, = v <p ",, and the equation of ideal gases

1 '- ^ P- ⁰ T - * - * ^ 1

Te or with Ac = A, = Ak

Vl · - Γ Λ. ir I to ίο - / * T-) <fi- 1

From the measurement of the state variables of the carrier medium (temperature T and relative vapor saturation φ) before and after the evaporation surface in the direction of the convective carrier flow, the evaporation mass flow density is determined.

The evaporative mass flow density measurement apparatus uses the measuring methods described above (singly or in combination). It consists of the following functional units:

- Preparation unit

- Measuring unit

- Evaluation unit.

The preparation unit contains the carrier gas reservoir with the necessary components for metering and temperature control of the carrier phase.

The measuring unit consists of the input and the output mixing chamber, as well as the flow channel with the measuring surface. The evaluation unit contains all necessary electronic control and measuring assemblies including a measuring computer.

In the preparation unit, the provision of a certain volume of the carrier medium with defined thermodynamic properties takes place. In this case, the carrier gas can be carried as a supply in the measuring device constantly or else originate in special A lwendun ^ falls from the environment. When the measurement is initiated, the measuring volume flows from the preparation unit into the measuring unit, where the contact between the carrier gas and the evaporation steam flow is established.

The state change of the carrier medium is measured using the integrating method at the times t and t + t _m in the input mixing chamber, wherein the constant carrier gas volume in the time t _m circulates in the flow channel.

By changing the flow path can be measured by the differential method. For this purpose, after the preparation of the measuring volume in the preparation unit and the initiation of the measurement, the carrier medium is guided once through the measuring unit over the evaporation surface and then into the environment. After stabilization of the flow state, the carrier gas state is measured in the input and output mixing chamber. The evaluation unit takes over the control of the measurement regime and after triggering the measurement all accumulated measurement data. According to equation (3) or (6) is then determined by the measuring computer found for the measurement conditions value of the evaporation mass flow density.

Figure 1 and Figure 2 illustrate the measuring principle for determining the evaporation mass flow density according to the integrating or differential method.

Figure 3 shows a device in which can be measured by deflecting the flow paths both after the integrating, as well as the differential variant. The preparation unit VE contains the carrier gas reservoir with the charging valve V1 and the metering valve V2. Furthermore, a heater H is provided for controlling the temperature of the support medium. In the measuring unit ME, the mixing chambers MK1 and MK 2 are accommodated with the temperature and steam sensors for measuring the carrier gas state, the flow channel SK with the contact surface Ay and the feedback realized via the valves V3 and V5. Furthermore, located at the entrance of MK 2, a second metering valve V4 and at the output of the measuring unit, the discharge valve V6. The evaluation unit AE consists of the modules for valve control, Trägergastemperierung, as well as from the temperature and Dampfkonzentrationsmeßschaltungen and the measuring computer. When starting the measuring process, the previously loaded via V1 or in VE constantly located carrier gas volume is passed after appropriate temperature control via the controllable metering valve V2 in the mixing chamber MK 1. In the case of integrating measurement, the initial state is determined here, and the input state is determined in the case of differential measurement. The carrier gas then passes through the flow channel SK, accumulates via Ay with the vapor stream of the evaporating phase, passes via V4 into the mixing chamber MK 2 and then through the integrating process via V5 into the recirculation (V6 closed) or during the differentiation process via V6 in the environment (V5 closed). The recirculated carrier gas flow is fed back into the mixing chamber MK 1 in the integrating measuring method via the valve V3 which is only opened in this variant. V2 is now closed, and via A, a circulation forms during the measuring time. After the measuring time has elapsed, the final state is determined by the sensors in MK 1. The sensors in MK 2 are deactivated in the whole time. In the differential method, the initial state is detected before the outflow of the carrier gas into the environment from the sensors in MK 2. All measured data are transferred to the evaluation unit AE for further processing.

Figure 4 shows a measuring device for operational use in the 3auwesen. The evaporation meter serves to determine the evaporation mass flow density of capillary water bound in porous building materials and is intended for measurement on a real structure. The carrier medium ambient air is used. The device operates on the differential method. The measuring device is placed with the measuring surface Ay on the surface of the building material. After switching on, the operating modes are selected:

- Automatic / constant

- Automatic / variable

- manually.

In "Automatic" mode, the measuring computer takes control of the measuring process Preparatory phase, the processor controls a driven by an electric motor M propeller LS to produce the Forced convection to the lowest speed. The resulting flow velocity v _k in cross-section Ak of Flow channel takes the calculator from a stored calibration function for engine power and Convection velocity. The air stream drawn in via the inlet nozzle E is divided in the intake channel. Only before the Propeller both streams are reunited, so that a disturbance of the further flow through other Air movements is excluded. Subsequently, the air flow passes through a heating grid, stabilizes and is over a

sharp edge turbulent in the mixing chamber MK 1 out. Here, a gas humidity and a temperature sensor measure the

Input variables before the air flow sweeps over the evaporation surface. Along the Meßflärhe reduces the Channel cross-section, and the flow rate increases. This can partially a Meßwertverfälschung by the Moisture enrichment of the air flow can be compensated. A sharp edge at the end of the Meßf laugh directs the air flow Initial value determination in the mixing chamber MK 2 and then via the exhaust nozzle A to the outside. After a selectable stabilization time for the measurement state, the measurement is triggered. The subsequently accordingly Equation (6) calculated value of the evaporation mass flow density is by comparison with the Meßwertauflösung on his Usability tested. If the result of the comparison is positive, the actual measuring program is started now, ie. H. it will be one Measurement series recorded under constant measuring conditions (operating mode constant) or under freely selectable

variable conditions (variable mode). The measurement result is immediately or after a statistical preparation of the

Data is displayed and stored in a memory. If the result of the comparison is negative, the The state of the air is changed so that the evaporation of the capillary water, which is also dependent on it, increases (first by rising

the convection rate, then by increasing the air temperature). The measuring computer allows the

Conversion of the measurement result to the initial air condition. In the operating mode "manual" convection speed and air temperature, as well as the expiry of the measuring program

chosen by the operator.

To exclude condensation phenomena, all duct sheathings are designed to be heat-insulating. The sealing of the measuring surface against the environment is done with a soft rubber. All electronic functional groups are housed above the flow channel and over the Stauertableau

operated on the top of the device.

The block diagram of the control and evaluation circuits is shown in Figure 5.

Claims (2)

1. A method for measuring the evaporation by measuring thermodynamic state variables, characterized in that the physical quantity - evaporation mass flow density - on the change in state of a gaseous carrier medium using the continuity equation from the continuum mechanism is determined, wherein the carrier medium penetrated by the vapor stream of the evaporating phase measuring surface swept over tangentially. The state change of the carrier medium is measured either simultaneously at two different locations or at two different times at a location of the measurement area. 2. Apparatus for measuring the evaporation characterized in that a preparation unit for the defined change of the thermodynamic state of a gaseous carrier medium and the formation of a defined flow state thereof and then a measuring unit consisting of the flow channel for receiving the vapor stream of the evaporating phase and from each one Flow channel upstream and downstream mixing chamber for measuring the change in state of the carrier medium, and an evaluation unit are arranged.