GB1571303A - Mass flow measurement - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO MASS FLOW MEASUREMENT (71) We, HAWKER SIDDELEY DYNAMICS ENGINEERING LIMITED, a British Company of Manor Road, Hatfield, Hertfordshire AL10 9LL, England, do hereby declare the invention for which we pray that a Patent may be granted to us, and the method by which it is to be performed to be particularly described in and by the following statement :F This invention relates to thermal devices for the measurement of fluid mass flow and, in particular, to devices which respond to the resolved component of a flow vector.

To obtain the total flow of a fluid across a section of a pipe or duct a velocity measurement is often made with some such device as a pitot tube placed at the centre of the section. The flow through relatively long straight pipes or ducts can be satisfactorily measured by this method where the flow is everywhere parallel to the pipe or duct axis, but a difficulty arises where the direction of flow is not normal to the section, as for example in irregularly shaped ducts or near an opening in a duct. The difficulty is that the total flow through the section is that effectively passing normal to the section, which means that the pitot tube or other transducer utilised for the measurement should be capable of resolving the velocity vector in the normal direction, which is a function generally not performed by conventional transducers.

In cases where mass rather than volumetric flow measurement is required the difficulty is further compounded since unless additional means are adopted for measuring fluid density the transducer must respond to the mass velocity vector component resolved in the normal direction; that is, it must have an output proportional to m cos + where m is the local mass velocity and ç is the angle between the local direction of flow and the normal to the sectoon. "Mass flow " generally means the rate at which mass crosses a section (i.e. the product of density, velocity and area) and "mass velocity" is here understood to mean the mass crossing the section per unit time per unit area, or simply the product of density and velocity.Thus the total mass flow through a duct may be derived from the distribution of mass velocities at different points over the section, obtained in a manner known to those skilled in the art, and the sectional area of the duct.

The magnitude of mass velocity can be measured very conveniently by means of thermal devices, which have an inherent response to convective heat transfer. Such devices, which commonly utilise the effect of the fluid velocity and density on the heat loss from a body suspended in a fluid stream, do not normally yield accurate information relating to the flow direction.

It is an object of the present invention to overcome the stated limitation of conventional devices and to provide an arrangement whereby a thermal device responds to the direction, in addition to the magnitude, of the mass flow of fluids such as air, natural gas or fuel oil.

According to the invention, apparatus for the measurement of fluid mass flow comprises an upstream electrically-heated body in the form of a frustum disposed with its axis perpendicular to the section of a duct through which the fluid flows; a downstream unheated body coaxial with the first and thermally insulated from it; and means for detection of the temperature difference between the two bodies.

In a preferred form the two bodies each comprise a conical frustum of similar size.

The transducer is of a class described in the Complete Specification of our Patent No. 1 515 302. It was disclosed therein how cylindrical bodies could be utilised in the measurement of oil burner combustion air mass flow parallel to their axes, which gives adequate accuracy for some types of burner. It is a further object of the invention to provide an arrangement whereby the normal component of mass flow can be measured more accurately than by the means of the previous patent.

One arrangement according to the invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings in which: Figure 1 is a general pictorial view of the transducer, Figures 2a and 2b show in cross section the internal arrangement of the heated and unheated bodies, and Figure 3 is a block schematic representation of the transducer and its associated electronic circuits.

Referring to Figure 1, the mass flow transducer comprises a heated conical frustum 1 disposed with its smaller flat circular end facing generally upstream, an unheated conical frustum 2 disposed with its smaller flat circular end facing generally downstream, a cylindrical thermal insulator 3, on either end of which bodies 1 and 2 are clamped, and a support tube 4 adapted to hold the frusta 1 and 2 so that their common axis is perpendicular to the section through which it is required to measure mass velocity.

Each body 1 or 2 is composed of copper or other relatively good thermally conductive material with an insert 5 or 6, respectively, of similar material, fitted inside the body. Insert 5 contains a heater resistor 7 and platinum resistor 8 while insert 6 contains only a platinum resistor 9. Good thermal conductivity material 10 is used to fill the gaps between all components within bodies 1 or 2. Electrically insulated connecting wires 11 from all electrical components are taken out through the base of each body 1 or 2 through a bore within insulator 3 and through the bore of tube 4.

To obtain signals representative of fluid mass flow from the transducer, its temperature-detecting resistors 8 and 9 are connected into the electronic circuit shown in Figure 3. In this circuit, the resistors 8 and 9 are connected in series with a constant current source 12 and across the input of a differential amplifier 13 the output voltage of which will then be proportional to the difference Ae in temperature between the heated and unheated bodies 1 and 2, respectively. An unstabilised electrical supply 14 is applied to the heater resistor 7 and a high-stability resistor 15 connected in series, the resulting potentials across the respective resistors providing inputs to a multiplier 16, which then has an output proportional to a product of voltage and current, or say power Q, dissipated by the heater resistor 7.Applying the outputs of amplifier 13 and multiplier 16 to the divider 17 gives a signal output from the latter proportional to the thermal conductance C of the boundary layer between the fluid and heater body 1, since Q=C.M and ns is the temperature difference across the layer.

Now, for a given flow direction C is a function of mass velocity and the temperature-dependent fluid characteristics of thermal conductivity and viscosity.

Accordingly, a signal proportional to fluid temperature is taken from the unheated resistor 9 and, with the signal proportional to C, applied to a shaping circuit 18. The functions bf circuit 18 are to linearise the relationship between mass velocity and thermal conductance where the velocity range is relatively wide, and to compensate for temperature where fluid temperature variations are considerable. All the individual electronic components or circuit blocks employed are of a conventional kind wellknown to those skilled in the art.

It has been found in this arrangement that, when the contours of the heated and unheated bodies 1 and 2, respectively, are generally conical and that of insulator 3 generally cylindrical, a mass flow signal is obtained which falls away from a maximum (when the flow vector is parallel to the transducer axis) as a cosine of the flow vector angle with respect to the transducer axis.

In the typical arrangement shown, giving good conformity with a cosine characteristic over a vector range within 30 of the transducer axis, the cone semi-angle is 10 * the frustum base to height ratio is 3 :2 and the length of the insulator 3 is approximately equal to its diameter. However conformity with the cosine characteristic over a vector range up to about 50 may be obtained by small changes to the aspect ratio of each frustum 1 or 2, their cone angles, or the radius of the leading edge of body 1.

It will be further appreciated that such a transducer not only provides a true measure of mass velocity where the flow vector is not normal to a duct section but also lends itself to us where the velocity distribution over the section varies with time or operating condition. In this case a plurality of such transducers is disposed so as to sample local mass velocity components over the section and thus enable a summation of total mass flow through the duct to be made.

WHAT WE CLAIM IS:- 1. Apparatus for the measurement of fluid mass flow comprising an upstream electrically-heated body in the form of a

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (10)

1. **WARNING** start of CLMS field may overlap end of DESC **.

   mass flow parallel to their axes, which gives adequate accuracy for some types of burner. It is a further object of the invention to provide an arrangement whereby the normal component of mass flow can be measured more accurately than by the means of the previous patent.

   One arrangement according to the invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings in which: Figure 1 is a general pictorial view of the transducer, Figures 2a and 2b show in cross section the internal arrangement of the heated and unheated bodies, and Figure 3 is a block schematic representation of the transducer and its associated electronic circuits.

   Referring to Figure 1, the mass flow transducer comprises a heated conical frustum 1 disposed with its smaller flat circular end facing generally upstream, an unheated conical frustum 2 disposed with its smaller flat circular end facing generally downstream, a cylindrical thermal insulator 3, on either end of which bodies 1 and 2 are clamped, and a support tube 4 adapted to hold the frusta 1 and 2 so that their common axis is perpendicular to the section through which it is required to measure mass velocity.

   Each body 1 or 2 is composed of copper or other relatively good thermally conductive material with an insert 5 or 6, respectively, of similar material, fitted inside the body. Insert 5 contains a heater resistor 7 and platinum resistor 8 while insert 6 contains only a platinum resistor 9. Good thermal conductivity material 10 is used to fill the gaps between all components within bodies 1 or 2. Electrically insulated connecting wires 11 from all electrical components are taken out through the base of each body 1 or 2 through a bore within insulator 3 and through the bore of tube 4.

   To obtain signals representative of fluid mass flow from the transducer, its temperature-detecting resistors 8 and 9 are connected into the electronic circuit shown in Figure 3. In this circuit, the resistors 8 and 9 are connected in series with a constant current source 12 and across the input of a differential amplifier 13 the output voltage of which will then be proportional to the difference Ae in temperature between the heated and unheated bodies 1 and 2, respectively. An unstabilised electrical supply 14 is applied to the heater resistor 7 and a high-stability resistor 15 connected in series, the resulting potentials across the respective resistors providing inputs to a multiplier 16, which then has an output proportional to a product of voltage and current, or say power Q, dissipated by the heater resistor 7.Applying the outputs of amplifier 13 and multiplier 16 to the divider

   17 gives a signal output from the latter proportional to the thermal conductance C of the boundary layer between the fluid and heater body 1, since Q=C.M and ns is the temperature difference across the layer.

   Now, for a given flow direction C is a function of mass velocity and the temperature-dependent fluid characteristics of thermal conductivity and viscosity.

   Accordingly, a signal proportional to fluid temperature is taken from the unheated resistor 9 and, with the signal proportional to C, applied to a shaping circuit 18. The functions bf circuit 18 are to linearise the relationship between mass velocity and thermal conductance where the velocity range is relatively wide, and to compensate for temperature where fluid temperature variations are considerable. All the individual electronic components or circuit blocks employed are of a conventional kind wellknown to those skilled in the art.

   It has been found in this arrangement that, when the contours of the heated and unheated bodies 1 and 2, respectively, are generally conical and that of insulator 3 generally cylindrical, a mass flow signal is obtained which falls away from a maximum (when the flow vector is parallel to the transducer axis) as a cosine of the flow vector angle with respect to the transducer axis.

   In the typical arrangement shown, giving good conformity with a cosine characteristic over a vector range within 30 of the transducer axis, the cone semi-angle is 10 * the frustum base to height ratio is 3 :2 and the length of the insulator 3 is approximately equal to its diameter. However conformity with the cosine characteristic over a vector range up to about 50 may be obtained by small changes to the aspect ratio of each frustum 1 or 2, their cone angles, or the radius of the leading edge of body 1.

   It will be further appreciated that such a transducer not only provides a true measure of mass velocity where the flow vector is not normal to a duct section but also lends itself to us where the velocity distribution over the section varies with time or operating condition. In this case a plurality of such transducers is disposed so as to sample local mass velocity components over the section and thus enable a summation of total mass flow through the duct to be made.

   WHAT WE CLAIM IS:- 1. Apparatus for the measurement of fluid mass flow comprising an upstream electrically-heated body in the form of a

   frustum disposed with its axis perpendicular to the section of a duct through which the fluid flows; a downstream unheated body coaxial with the first and thermally insulated from it: and means for detection of the temperature difference between the two bodies.
2. 2. Apparatus according to claim 1, wherein each of said bodies comprises a conical frustum, the upstream body having its smaller end facing upstream and the downstream body having its smaller end facing downstream.
3. 3. Apparatus according to claim 2, wherein the two conical frusta are united and supported by a cylindrical insulator secured between their larger ends.
4. 4. Apparatus according to claim 1 or claim 2 or claim 3, wherein the upstream body contains an electrical resistance heater and a temperature-responsive resistor in good thermally conductive relationship with the external surface of the body, and the downstream body contains only a temperature-responsive resistor likewise in good thermally conductive relationship with the external surface of the body.
5. 5. Apparatus according to claim 4, wherein the two temperature-responsive resistors are connected in series across both a constant current source and the input of a differential amplifier, whereby a signal representing the temperature difference betwen the two bodies is obtained at the amplifier output, and the heater is connected in series with a high-stability resistor across an electrical supply to provide inputs to a multiplier that gives on its output a signal proportional to the power dissipated by the heater.
6. 6. Apparatus according to claim 5, wherein the signal outputs from the differential amplifier and the multiplier are applied to a divider to derive a signal proportional to the thermal conductance of the boundary layer between the fluid and the heated body.
7. 7. Apparatus according to claim 6, wherein the output signal from the divider is combined with a signal proportional to fluid temperature taken from the temperature-responsive resistor in the unheated body to linearise the relationship between mass velocity and thermal conductance and compensate for fluid temperature variations.
8. 8. Apparatus according to any one of the preceding claims, wherein the cone semiangle of each body is in the order of 10 and the base to height ratio is in the order of 3:2.
9. 9. Apparatus according to claim 8, wherein the insulator has a diameter equal to the larger end diameter of the conical bodies and a length approximately equal to its diameter.
10. 10. Apparatus for the measurement of fluid mass flow, substantially as described with reference to the accompanying drawings.