Smart Air Supply Anti-Haze masks

Xiang Zheng Tu 

A smart air supply anti-haze mask, as shown in the above figure, comprises a sealed mask, a pollution filter, a micro-electric fan and a POSIFA thermal flow sensor based microcontroller. The mask is sealed to the face during inhalation and creates a breathing space by resting far away from the face. Two one - way valves are connected to the mask which are used to direct air flow in and out respectively. The pollution filter is made of multiple porous membranes and blocks against haze PM2.5 particles in the suctioned air.

The micro-electric fan moves enough filtered air to the mask through the in air flow valve.

The air is required to deliver to the mask according to an air flow waveform that is restored in the microcontroller. In order to do so the micro-electric fan is driven by a PWM signal that is send from the microcontroller. The PWM signal is generated by modulating an air flow rate signal measured by a thermal flow sensor. The thermal flow sensor can be installed in two ways. One is installed on the bask surface of the filter. In the first way the air flow rate is measured by the sensor immediately after passing the filter. In the secondly way the air flow rate is measured by the sensor immediately after the fan blowing.

In the second way the thermal flow sensor is installed in a laminar flow restrictor. Reference to the above figure, the restrictor is positioned in an air flow tube that is located between the micro-electric fan and the in one-way valve. The micro-electric fan produces a turbulent flow to the air flow tube. The restrictor consists of a plurality of collimated channels which are used in dividing the velocity components of the incoming flow stream into smaller components. Some of the velocity components cancel each other thereby presenting a more uniform velocity profile, reducing the turbulence of the flow, and allowing laminar flow passing through the channels.

As well known, laminar flow occurs at low Reynolds numbers, where viscous forces are dominant, and is characterized by smooth, constant fluid motion. For air flow in a channel, the Reynolds number is defined as

Re = (ρvD_H)/μ  = (QD_H) / (NυA)                        (1)

where:

Re is 2300 for air flow.

D_H is the hydraulic diameter of the channels (m).

Q is the volumetric air flow rate (m³/s).

A is the channel's cross-sectional area (m²).

N is the number of channels.

v is the mean velocity of air flow (m/s).

μ is the dynamic viscosity of air equaling to 1.983x10^-5 Pa·s.

ν is the kinematic viscosity of air equaling to 15.11x10^-6m²/s.

ρ is the density air equaling to 1.2754 kg/m³

The hydraulic diameter of the channels can be found by

D_H = 4A/P                                                             (2)

Where A is the cross-sectional area of the channel and P is the total perimeter of all channel walls that contact with the air flow. It should be noted that the length of the channel exposed to the flow is not included in the Reynolds number.

In the second way the air flow passes through a porous material such as a pollution filter. In this case Darcy‟s law is applicable which is stated as

Q = - (κAΔp) / (μL)                                         (3)

Where:

Q (m³/s) is the total discharge,

κ (m²) is the intrinsic permeability of the porous material,

A (m²) is the cross-sectional area to flow,

Δp (Pa) is the total pressure drop,

μ (Pa·s)  is the viscosity, and

L (m) is the length. 

Darcy‟s law is only applied for Re < 1, although it is sometimes not easy to define the pore diameter in a stringent way. Darcy’s law assumes laminar or viscous flow (creep velocity) and it does not involve the inertia term. Darcy’s law also assumes that in a porous material a large surface area is exposed to flow, hence the viscous resistance will greatly exceed acceleration forces in the flow.

So for a smart air supply anti-haze mask the thermal flow sensor is not necessary to install in a laminar flow restrictor. Instead it may directly install on the surface of the filter of the mask because the out flow of the filter is an air laminar flow. The laminar flow tends to flow without lateral mixing, and adjacent layers slide past one another. There are no cross-currents perpendicular to the direction of flow, nor eddies or swirls of flows.

As shown in the Darcy‟s law the total pressure drop (Δp) represents the viscous resistance to the flow. That is why the air flow is reduced while one wears a normal mask. There is no doubt that you want to protect you from air pollution and guard your health you will sacrifice some comfort or even though feel overly suffocating. But when wear a smart air supply anti-haze mask you will feel comfortable as usual and absolutely nothing will happen, because there is a micro electric-fan that can supply enough filtered air to you.